Each human race has different alien species "creators"
Basically they altered primitive human DNA to help speed up the evolution process.
Some of the aliens were bad and some good. The bad aliens are protecting their human relatives as are the good aliens.
The bad aliens are not only protecting their own but they are trying to eliminate their good alien human offspring.
This why you see specific races targeted for elimination. This why you see race mixing pushed for some races and not all races. If they can breed your ancestor blood out and destroy your bloodlines then so be it.