Hypothesize why only one genus and species remains in the hominin group

Modern humans have been around for a very short period of time. There are differences in race, asian people tend to be smaller, stuff like that. The problem is different races marry and those genes mix. If Asians only married pure blood Asians, Africans only married and procreated with other Africans thanks in hundreds of thousands of years different sub species would most likely start to occur. Seeing as how human population is all connected today, that won't happen.