The reasons for this curious finding aren’t fully known, although there are theories. The chief explanation is that the ancestors of Europeans (and most white Americans) suffered repeated population “bottlenecks” in which their numbers crashed as result of epidemics, catastrophes, and genocide. Each time that happened, the population lost a lot of its genetic diversity simply because a lot of people died.

The survivors, like their ancestors, carried a certain random collection of deleterious SNPs—genes that caused disease or increased the risk of disease. When the population rebounded, those genes were spread widely as the small number of survivors gave rise to all living descendants. But if they were potentially bad, why weren’t they flushed out by natural selection? That is the mystery. It’s possible that some deleterious SNPs were “dragged along” into the future because they were physically close on chromosomes to newly arising SNPs that increased a person’s biological fitness.
-(David Brown,The Washington Post)