No faster than light travel. Yeah, 3000 real-time years have passed, but subjectively, how long did it take to get there?
Justified in Shadow of the Giant; the colony ships were deliberately made monocultural as an attempt to encourage diversity in human culture.
It's also implied that humans have gone and colonized an assload of worlds in the three milennia prior to the sequels, allowing for a staggering array of very specific diversities. Thus, Lusitania was for people who just happened to enjoy the 20th-century Brazilian Catholic lifestyle.
Plus they were Brasilian.
Part of the reason is because they have very strict guidelines about what they can do because they're on a planet with the only known sentient life in the Galaxy other than the (as far as they know) extinct Buggers. The rules are to prevent cultural contamination or affecting their development as an intelligent species. They're only allowed to use a certain area of land and have to keep a small population. They can't build large buildings or use any overt displays of technology. The rules they have to live by keep their way of life pretty simple (although that doesn't explain the language).
In Speaker For The Dead, it's revealed that Pipo and his son were killed by the piggies because the piggies thought they were helping them become father trees, which is granted after they do something significant. As far as I remember, in both cases, the xenologers discovered the truth behind the life cycles of the piggies moments before their deaths, and the xenologers let the piggies vivisect them because they didn't want to "kill" the piggies they were friends with. But if this happened right after Pipo and Libo had (independently) discovered that "killing" the piggies meant them moving on to their third stage in life, why would they refuse to kill the piggies, sacrificing themselves instead? From a logical standpoint, it makes perfect sense to want to help the piggies, since it would let them reach their final stage of their life. But they refused to shed blood, instead letting themselves die, robbing the piggies of becoming trees and causing Libo (in the case of Pipo) and Novinha (in both cases) to be horribly crushed by despair. Sure, cutting open their friends would have felt like a bad thing, but they would have known that they weren't killing them, and dying and hurting the people who really love you seems like a much worse price to pay (sorry if that was kind of long winded and convoluted, I couldn't think of a better way to phrase it all).
Cutting open their friends would have felt like a bad thing, you are trying to assign a logical reason to an emotional decision: it's easier for them to die than to kill someone.
Also one of the points of the whole series was how disastrous results arise when people do not allow themselves to think in terms of a different context. Pipo and Libo made their choices based on their context as human beings and their Catholic upbringing. They weren't able to let go of a moral compass in their action despite the fact that it was inapplicable to the piggies. This is in contrast to Ender, who was raised to consider the perspective of another living being.
Ender later joined the Catholic Church on Lusitania and it could be revealed that he considers that moment especially hard to bear, having killed both Stilson and Bonzo unknown to him at the time of their deaths, finding out about it later after the Third Formic War. He was a reviled monster for killing off an entire species, countless military personnel in the Third Formic War, and two kids from his childhood. Purposefully killing a sentient being, even without a religious background, would be difficult, especially coming from that background.
I'm sorry, but having read this series several times, I think it's ridiculous how all problems could have been solved just by asking how the piggies reproduce. How hard is it to say, "I'm sorry, piggies, but we cannot turn into trees, please do not cut us"? Problem solved. It's incredibly dippy how that plot point was presented. Neither of these two men could say, "We don't turn into trues, do not kill us?" It's not like the Piggies cracked them over the head with a rock either. They could have gotten out of this. It's contrived. It seemed to have been written this way to make Ender come from the stars and save everyone.
Maybe it was. But the Watsonian reason was the Alien Non-Interference Clause put in place to prevent cultural contamination. As it happens, your exact complaint is echoed by an off-planet xenologer, who sends a snippy e-mail to Libo about how stupid it is that he (Libo) can't explain yet how piggies make babies. Libo responds, point blank, "We're not allowed to ask. There is a law." Ridiculously obstructive? Absolutely, and nobody denies it; OSC even lampshades it in the very same e-mail. But you asked about the reason, and there it is.
The fact that the piggies get really upset after they find out what they have done makes this whole thing even more stupid. By not telling them the truth, they just caused more problems for everyone. And there has to be some difference between "prevent cultural contamination" and "explain that we have different biology and reproductive methods". Also, the first guy was already a father, so it should have been easy for him to show them that they reproduced differently.
It doesn't help that people are incredibly inconsistent about their level of concern over cultural contamination. Miro and Ouanda just say screw it and start deliberately introducing new technologies in an attempt to up their standard of living, but they still don't ask about these basic things. Then there's the giant, never examined question of how they taught the piggies multiple human languages without this coming up almost immediately. (And why the hell people worried about cultural contamination would choose, of all ways to learn about them, teaching them their language and asking.)
It's also pretty hard to believe that after Libo's death neither Miro nor Ouanda asked why. They were both his apprentices; Ouanda was his daughter and Miro thought of him as a father. They really stuck patiently to rules they didn't even believe in and never once gave in to their grief and their need to know?
The fact that no one figured out the piggies secret and a way to get the piggies to live in peace with each other and with humans for more than 100 years before Ender set foot on their planet reeks of Mighty Whitey. I mean, come on, no one else in the entire colony was remotely curious about the Descolada or the lack of biodiversity or the piggies, even after they killed two men who were supposed to be ambassadors to their tribe? It seemed like a problem that no one had applied logic to before Ender got there, with the Unfortunate Implication being that the Brazilian-cultured colonists were being too emotional and not rational enough to examine their world correctly, but Ender with his superior American/White/Nordic culture was able to save the day with his rational prowess and ability to make more emotionally detached decisions, like when he "killed" Human. Also, why didn't anyone ever think of giving the piggies a written agreement to sign before?
You need to remember the strict rules placed on them: only observe, do not interfere. They couldn't give them a written agreement because they weren't allowed to introduce writing. And they were extremely curious about the Descolada, seeing as it was a plague on the entire village that killed hundreds of colonists. But the fact that it was a plague focused their curiosity on the curing aspect, not the biological function aspect. They also theorized the lack of biodiversity was caused by the descolada causing a genetic bottleneck. The only reason their theories moved forward after Ender's arrival was because his mindset was based on breaking the rules, while their Catholic culture places a huge taboo on breaking the rules.
Those are all perfectly good in-text reasons, but the original troper was critiquing author choices, not character choices. The author controls the text; this is fiction, he makes the decisions of how things are handled. And, yeah, with that in mind, the whole idea that these Brazillian people can't manage in 100 years what a white guy can in only a few reeks of Mighty Whitey and Unfortunate Implications. Yeah, there are limitations on what the Lusitanian colonists are allowed to do, but the author set those limitations on them to give Ender the win, and at the end of the day it's still a white guy landing on soil owned by non-white people to show them how their world works, because they apparently couldn't figure it out due to rules they were 'too stupid and emotional' not to question.
I think you're trying too hard. Remember that the presumably white offworld xenologers are doing even worse than the Lusitanians before Ender gets there. Ela mentions outright in Speaker that she's not allowed to get any biological samples from the pequeninos, or even from outside the fence, and the one biologist who actually probably could have figured it out before Ender (Novinha) shut down her inquiry (and her lover's and daughter's inquiries) for understandable emotional reasons. And given that the other major heroes of the Shadow series are the Afro-Greek Bean and the Armenian woman Petra, and most authors wouldn't even bother to specify that the entire planet's human population is black Brazilian...