Of course Ashkenazim tend to be lighter skinned that Jews in the Middle East. Most converts in Europe were quite pale, whereas most converts in the Middle East looked like the Jews in ancient times (who were from the same reason). An added factor might be that Jews with lighter skin and blond hair had a better chance surviving pogroms that darker European Jews.
The reason there are more Ashkenazim is that during the last few centuries the areas where Ashkenazim lived had a good economy and improvements in medicine, triggering a population explosion. Four hundred years ago the numbers were roughly equal. A thousand years ago, Ashkenazim were a small minority.
But in all fairness, people in the Middle East don’t look all that different than ones from Europe, and in general humans are pretty much similar. There is about as much genetic diversity among humans as among poodles (some dark, some light, some big, some small, but all poodles). This tends to support our view that humanity has only been around a short time.