Generally speaking, that's done because it makes it easier to find for people who want to see it. I always tag my videos with the ethnicity of the girls in them, so if someone is looking specifically for sistas or desis or whatever they don't have to sift through 300 videos just to find the two they want. You don't need to do that for white chicks because they're everywhere. lol
I actually make it a point to specify that I prefer to work with non-white girls when I put out casting calls. I try to mix it up, though; sometimes I won't say anything, sometimes I will... but I think it's made for a pretty diverse roster, if a little Asian-heavy.
In short, everything is a sub-category, because everyone likes different things, and some things are more popular than others.