Why there used to be so many Jews in Poland?

I would like to invite you to watch the video below and do your own research as well to answer the question in the heading.

There were plenty of Jews in Poland as We all know about Holocaust and German death camps where Hitler and his army brutally murdered millions of Polish Jews and Polish people.

The good question is, how come there were so many of them in Poland? We all know they did not have their own country at the time, but why they picked Poland over other countries in the world? This topic is very interesting and it can be a good lesson of a history for all of us.