This question arose in one of my masters history classes and had my professor stumped. Why, after being forced to leave Spain, did the majority of European Jews migrate to Poland and the former Soviet Union?
Simply put, they didn't. The Polish/Russian Jewry tended to be formerly German Jews pushed east or fleeing east to the relative tolerance of the Polish-Lithuanian Commonwealth, especially before the 17th century when that tolerance began to wear thin. Spanish Jewry, while it did flee all over, largely went to North Africa (Morocco/Algeria/Tunisia) or to the Ottoman Empire (usually Salonika/Thessaloniki and other parts of Greece, some in Turkey proper, others in Egypt or Palestine). You could find Sephardic Jews, that is, Jews with Spanish expulsion ancestry, in other parts of Europe like the Netherlands, but they were a distinct minority of European Jews.
As Mark Mazower pointed out in his book on the city of Thessaloniki, "Salonika: City of Ghosts," up to the late 19th century Salonika was actually plurality Jewish, and that community was essentially entirely Sephardim.
According to the book "Poland" by Adam Zamoyski the Polish Lithuanian Commonwealth had a lot of religious freedom and as Jose_ber has said it didn't really have the oppression that the more western states did.
Krakow was majority jewish and german for a very long time because of this movement of people.