Like most regionally-locked groups of people, whatever was more predominate among the original peoples is what gets expressed the most in genes.
I mean, why are people from one area typically shorter than people from another area? Just how genetics works. We have different skin colors because humans congregated into certain areas after the mass exodus from the homeland, and early physical features that were the most common, if the local civilization stayed local, just end up being what is most commonly witnessed even thousands of years later.
We're all the same, yet have different physical characteristics from each other if one is from a culture that has kept themselves "pure". Genetic freakshows happen all the time too, so that explains the random ones that don't fit the rest of the herd.
There are some truths to certain "racial" stereotypes because, well... that's just how the specific groups bred themselves over the course of history. Not on purpose, just that's how it happened. It survives today because some cultures still lock themselves to regions, or people naturally prefer to mate with other like individuals, so genetics don't get to get diluted all that often. Here in the U.S. it is the most mixed, at least if you ignore inter-racial marriages, of which I think England probably beats us, in a per-capita measurement at least.
Why we have bigger wangs than our primate brethren? Hell if I know.