It seems that after the expulsion of Jews from Jerusalem in antiquity, most Jewish population centers were located to the west of Israel (around the Mediterranean or in Europe), with the main exception being Mesopotamia, which already had established Jewish populations before the Roman exile. I'm well aware that were also some small Jewish populations to the east in Iran and India as well as to the south in Ethiopia and Yemen, but from my understanding, it seems that the bulk of the Jewish diaspora migrated westward from their homeland. Is there any historical evidence to suggest why this was the case?
Or is it the case that Jewish populations spread outwards from Israel rather evenly, but that hundreds of years of European colonialism and Eurocentrism have changed global Jewish demographics as well as our perception of them in antiquity? In that sense, I'm also curious as to why Ashkenazi Jews are by far the most populous Jewish ethnicity?
The Jewish population today is not the same as it was in the past.
There used to be large populations in Arabia, Egypt, and Persia. The Jews of Alexandria used to be one of the largest Jewish diaspora communities. The Arabian Jews were annihilated early on by the Muslims. Small remnants did live on in Yemen The other areas had a slower decrease. Those communities were only recently destroyed, mid 20th century. Before than, most large cities had Jewish communities. The Persian community still exists but it is a difficult situation. Most of the historic Jewish communities of the Middle east and north Africa are gone. They were kicked out of their indigenous countries and the people mostly immigrated to Israel. That is why most Israelis are of Sephardi or Mizrahi descent. Some who lived in French North Africa did move to France. The French Jewish community is now mostly descendants of refugees from French North Africa and not the historical population.
Historically, non-Ashkenazi communities were in the majority. It's only recently, 18th-19th century did this change. It was due to increased family sizes and potatoes. The Ashkenazi population did move over time from Italy to Germany to Poland to the US.
The diaspora did spread all over but relatively few communities have survived into the modern era. We also cannot deny the nutritional powerhouse called the potato.