Because they didn't want her to look scary with a horn coming out of her nose and forehead? That's why Barney doesn't have razor-sharp teeth and claws coming out of his hands and feet. It's a show aimed for toddlers.
And in that matter, how could B.J. and Riff be related to Baby Bop? They're clearly different species.
Help me out with the timeline. In many viewings of the show when I was a little boy, I've seen Baby Bop and B.J. be child-size, then adult-size. What is going on here? Is time somehow passing for them, yet not affecting the other kids and Barney?
BJ and Baby Bop both started out adult size, but shrunk in order to fit the character's ages better (seven and three, respectively). As for the interspecies dinosaur family thing, one can assume that BJ, Baby Bop, and/or Riff were adopted in a sense. Luckily, Barney doesn't have a family relationship to work with (in context, his friends are his family, as they bring him to life).
"Barney is a dinosaur from our imagination" that's how the theme song starts out. I think that clears up all your questions.
Personally, I've always thought of Barney as a Tyrannosaurus, B.J. and Baby Bop as Protoceratops and Riff as Saurolophus.
Word of God says that Baby Bop is a Triceratops, B.J. a Protoceratops, and Riff's a Hadrosaur.
Maybe I'm making this up, but wasn't there a time where Barney had these monstrously huge, human-like feet before they decided to shrink it to the size they are now? I remember seeing a Barney movie as a toddler, and seeing those feet creeped me out. Did they actually have something like this, or am I just making it up?
Confirmed. Definitely changed the feet size there.
I see. So they changed it because they realized it was too freaky for the kids? How long did Barney have those old creepy feet?
Why do people hate the show so much? A lot of people say it's because Barney looks creepy and he's always smiling. In that case, why isn't Big Bird or Mickey Mouse hated as bad? That never made sense to me.
The incredibly annoying voice might have something to do with it.
One of the problems with Barney is that it comes across as patronizingly dumb. Shows like Sesame Street talk to kids, Barney talks down to kids. Barney is Tastes Like Diabetes, while Big Bird is just sweet. Carroll Spinney himself has gone on record criticizing the Barney franchise for exactly this reason.
Another issue could be that all or most of the "Barney kids" are much older than the show's target demographic. Of course, this has occurred with other kids' shows, too. Gabi of Sesame Street, for instance, has appeared on episodes for years. But unlike Sesame Street and other such shows, there is no intellectual line between Barney and the little kids vs. older cast members. Thus, it's pretty jarring, at least from an adult perspective, to see kids that are clearly in their early teens acting like songs and activities aimed at toddlers are genuinely fun/intriguing. (For instance, the kids who played Luci and Michael in the first few seasons did not leave the franchise until at least age 14).
To go along with that: You also have to consider Barney has no Periphery Demographic. With shows like Sesame StreetWishbone, or Arthur, for instance, there are plenty of things for adults to enjoy. Parodies, pop culture references, and references to works of literature and film meant for older audiences abound, even if the target audience doesn't "get it." With Barney and Friends, it's just teenagers dancing around with a guy in a dinosaur suit. As others have said, patronizingly dumb.