Because racists are terrified of the idea of non-whites making up a larger percentage of the population than they do currently.
But is doing that downvoting likely to increase or decrease the chances of votes for Trump ? I'm pretty sure it's increasing those chances.
So that brings the question ... are these people stupid ? Not wanting to discuss (and hopefully fix) the reason Trump got elected is not going to help.
Or, I guess, perhaps they're Russian.