WMG / Monsters vs. Aliens (2013)

The reason Insectosuarus is absent from the TV series is because...

He's dead.
With a lot of Moth, Butterfly, and insect species in general, they can spend a majority of their life cycle in the pupa stage, while the mature, adult stage is a relatively short period of a day to a week breeding like mad before they die. Poor Insecto, being one of a kind, probably missed out on going Out with a Bang.

• A more likely reason is that his size made him too expensive to animate on a TV series budget, which was probably the reason Susan was given the ability to shrink herself.
  • Out-of-Universe, that's exactly the reason.

Alternately, Insectosaurus became the Hero of Another Story when one of America's allies requested that she be stationed in their country to fend off the recurring attacks by various giant monsters over there; had the series continued, she'd have been revealed as the first member of a second monster-based team.

In the TV series, there are no plans for an Alien Invasion by Coverton's species

Much like the titular alien from Invader Zim, Coverton wasn't sent to Earth to prepare for an alien invasion, but instead sent to Earth to get rid of him because of his incompetence. Much like Tallests Red and Purple, the Grand Coverlord keeps taking his calls to keep up the illusion and make sure he doesn't return home. They also accepted his request to have Sta'abi assigned there because it also kept her out of the way.

All Area 50-something's earthlings (except for B.O.B., of course) actually knows about Coverton's real intentions.

Seriously, Coverton doesn't take any effort to preserve is... well, cover. How long it takes before someone understands that there's something wrong with that guy?! Susan, Monger and the others pretend to not have realized yet, so to keep Coverton stuck on Earth, in a secret base, where they can keep an eye on him and acquire vital information on the invaders' plans.

Coverton is not the only alien invader.

In case of Sta'abi, this is pretty obvious, given that she is an expy of Klingon. Her species probably purposefully sent the Vornicarn "egg" to Earth. Also, she has the word "stab" in her name.

In case of Sqweep, well... he was going to make an essay on Earth's dominant species, right? He actually thought that the dominant species were squirrels, even though there is a lot of human-made structures near. His species, who obviously hail from a Utopia, were aiming for the weakest sentient species possible. It was close enough.

Basically, they are all in an Anti-Human Alliance, and they want humans as slaves and cattle.

• One problem with that theory, none of the others except for Coverton seemed to purposefully do anything to harm the monsters or the Earth.

It is in the same universe with American Dad!

One of the most obvious reasons: look at Coverton and don't say he doesn't look like Roger.

Also, Coverlord is not the leader of his people; he's just the colonization commander to the real leader Emperor Zing, and Coverton just really admires him. However, Coverlord is also one of the saner people on Roger's planet, and his troops might be even incompetent, thus Emperor Zing offs him. Simultaneously, President Hathaway is no longer president, and, since he's the only reason Coverton is still on Earth, Coverton is captured and dissected. Since Emperor Zing is a sex-obsessed dick, he doesn't care about Coverton at all.

Had the series continued long enough, Coverlord would have been revealed to be Gallaxhar.

Considering we never see what Coverlord actually looks like, it's entirely possible that had the series continued, he would have turned out to be Gallaxhar himself (or a clone), having survived the explosion and went into hiding.