I think it's because of color and power that one has over the other. It is something that has been there but has presently suppressed. However, it should be the time when people should realize that we are all human beings who were created by one God.
A lot of folks don't like feeling challenged or the unknown. So if they are not educated about certain cultures (real education, not just what folks say) they may use slurs and act superior. Or if they do not understand a person's lifestyle (say genderqueer for example) they may also lash out. Other folks just need to have power, to be see as an authority and be respected-- which tends to mean acting just as they do to have their respect returned
We ARE all different. The main problem is culture. Racists target people because they live differently, and they identify these people primarily through the colour of the person's skin. While we are all different in the way we live and our cultures, there's no excuse for hating other people because they're not like you. That's my opinion on why they do it, anyway.
In what way do you mean they are being treated differently? You must mean differently in some bad ways, to have asked this question. Certainly different treatment isn't always improper . We are all different, sometimes quite dramatically so. Perhaps you mean differently in a disrespectful way having nothing to do with how they treat you.
We are all human, but we are different. Modern science has found that modern humans have gene flows from (homo sapiens), neanderthals and denisovians. . Read rise of the homo sapiens on this link. http://en.wikipedia.org/wiki/Human . That is still no reason to treat another human race differently.