If Polish Jewry as well as German Jewry was facing grave problems in the 1930s, why didn't Polish Jews flee Poland at as high a rate as German Jews fled Germany? Because the problems facing Polish Jews were different from those facing German Jews?
Coming to think of it, two possible answers to my own question might be the following:
The regime in Poland in the 1930s wasn't as explicitly and murderously anti-Semitic as was the case in Nazi Germany.
Many more Polish Jews were poor than German Jews were, so that they wouldn't have had the means of leaving Poland even if they wanted to. Also, many more Polish Jews than German Jews were traditional, so that they wouldn't have wanted to leave Poland for places like the United States that they regarded as "impure" or something like that.
Does that make sense?