Well, I suppose it's because they already have a method that works, and they don't want to mess around with it. Like the game developer said, there's simply more demand (I think) for a game that focuses on humanoid characters. When you're sinking millions of dollars in capital into a game, you want to make sure you at the very least recoup your investment, and you can usually do that by sticking to what's mainstream.
I understand what you mean about putting it in, but I think that most don't want to do it because it's put a degree of asymmetry into a game, something MMOs don't tend to do well. It's also a measure of how much work a developer puts into the game vs. the result they get. If they have to come up with new ideas solely for the animal side, they get half as much done for the same mount of work. (example, if they make an epic item for a humanoid, they have to spend the same amount of time making an epic item for the animal class as well)
That being said, I've seen a few more independent MMOs come up with the animal class idea. One, I was really interested in was Trials of Ascension. Unfortunately they went bankrupt, but before that they wanted to introduce a variety of races, including dragons. Interesting concept. But the thing is, they failed to attract large scale investment, and there were just too many angles to cover; so it never got released.
So, yeah, it'd be cool, but thing is, I doubt any of the developers would see it as cost-effective.
