Why are there different races of humans?

The different varities of humankind that are classified as a 'race' were genetic adaptations to environmental factors that provided a genetic advantage over others that did not exhibit the adaptation. Just as 'races' are somewhat different from each other in appearance, there are other genetic traits and dispositions to certain diseases or resistance to certain diseases that are reflected statistically. These would also be due to genetics.

From a Biblical perspective, there are not different races of humans on Earth. There is only one race. The Bible says in Acts 17:23 And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation; So you can see it is clear God made us one race, and that was from Adam, the first man.