Because they were run out of much of Western Europe during the Middle Ages and somewhat tolerated by the Blyat Empire, where they merely suffered pogroms and were forced into the Pale of Settlement.

Ashkenazic Jews or Ashkenazim,[a] constitute a Jewish diaspora population that emerged in the Holy Roman Empire around the end of the first millennium CE.[8] They traditionally spoke Yiddish[8] and largely migrated towards northern and eastern Europe during the late Middle Ages due to persecution.

https://en.m.wikipedia.org/wiki/Ashkenazi_Jews

The "Poland-Lithuania" that most of these migrations went to, at the time included large portions of present day Ukraine, Belarus and Russia. The regions with the highest Jewish populations prior to WW2, seem to match the borders of the polish-lithuanian commonwealth before it was partitioned by their neighbours in the 18th century. https://en.m.wikipedia.org/wiki/Partitions_of_Poland