Does anyone know why there were so many Pollack jokes?

back when it was socially acceptable to tell ethnic jokes? I spoke to a man last month that was of Polish descent and when I asked him about it, he gave me a very intriguing answer. I was wondering if anyone else ever was curious about it, or if they knew the reason. He explained that it was because many Polish immigrants didn’t know English, and were treated with disdain for it, even though many were very intelligent and spoke other languages, but that English wasn’t one of them.