A surprising amount of racism still exists in the adult industry, and it's not always the girl.  You mentioned that it would cut down on a lot of work for them, but it may be just the opposite.  It's rumored that a girl who does interracial scenes becomes less desireable to a lot of producers, so in fact they might potentially lose more work by doing interracial... so the fact that a girl just starting in the business and trying to establish herself doesn't do interracial scenes doesn't necessarily mean SHE'S a racist... but that the people who hire her are.

You are probably right about not doing anal limiting their job prospects... but again, since a lot of the talent on these model sites are newbies to the porn business, they don't do anal scenes (they've probably never done it in their private lives).  But you'll notice that after they've done a few videos, they eventually get around to it.  They get work initially because they're a new face, but once they're no longer the flavor of the week, they find that producers no longer want to hire them if they won't allow Greek, so suddenly you see them touted on the box cover "So-and-so in her first anal scene"

As always, money talks, bullshit walks...