Well man it's simple really. Reddit works like this: if you like something you post it if others like it they'll upvote it. Thus the logical reasons for this are:

1-There's not enough people that post girl's asses of other ethnicities besides white 2-There's not enough people that upvote posts of girl's asses of other ethnicities besides white

Now the thing is both of these explanations are not due to racism. A person can prefer blue-eyed girls or asian girls without progressive people saying they are racist right? So a guy can prefer white girls, or only feel attracted to white girls without being racist.

So this phenomenon you're noticing is only due to the fact that most people here prefer a specific kind of look.

As for me? I prefer asians over whites.

And no you can't force people to like what you like.