We all have the same homo-sapian ancestor, so why are humans so diverse?
Over the course of evolution genome and epigenome have interacted with environment driving speciation (formation of new species) as well as variation in particular species. Races are the likely outcome of such quantitative and qualitative interactions.
Human beings as well as animals are attracted towards morphological variations, for instance a Caucasian person might find an oriental partner more attractive. Such mating (biological term for marriage or union) would lead to genetic variations and rise of mixed race that may stabilize to a newer race over the time.
Laboratory mouse (mus musculus) is only one species yet there are many coat-colors!
Likewise human races have evolved in response to natural and man-made(mating) factors! Morgan has very nicely explained some of these factors.
Hope this additional information and example will be helpful as well.