Try /r/askanthropology as this is a more general situation than just that group in that city.

So there are two general rules that govern where people immigrated to in the US. The first was a tendency to find like communities. For example, a bunch of people from Italy settled in New York and Philadelphia, and their presence encouraged other people from Italy to settle in the same area. First of all, some people would actually know people who had emigrated to the US in their own family or village. Second, familial and familiar ties, especially when surrounded by so many people from other cultures when people had largely lived in isolated homogeneity for millennia, encouraged people to support kin and neighbors in moving to the United States. "My cousin's husband's brother went to New York two years ago. You look him up. He'll help you out."

Secondly, once even a small group of people had located to one area they began to provide services that others from the same area would find attractive. For example, churches dedicated to saints from the homeland with priests who spoke your native language, shops that sold the foodstuffs you were raised eating, and newspapers in your native language.

The second thing is that people tend to go places that remind them of home. You would think that, for example, Scots who spent hours hiking up forbidding hills and trying to farm in the highlands would have wanted to move somewhere flat, or that Scandinavians, who came from places with long, cold winters would want to live somewhere warm. But the truth is the exact opposite. People go where they feel used-to-it-comfortable, even if what they are used to is decidedly uncomfortable. This is why so many Swedes wound up in Minnesota and Wisconsin and so many Vietnamese from the Mekong Delta area wound up in Texas and Louisiana.

So once Polish immigration got going in Chicago, it would tend to keep going. But as to why Chicago in the first place, well that has a lot to do with when it started. The mass of early Polish migration took place in the middle of the 19th century, shortly after Poland was divided up by Russia, Prussia, and Austria-Hungary. There was an effort to exterminate Polish culture and language, and many Poles fled for the New World. And at the time, Chicago was a booming metropolis with all kinds of new innovations and businesses growing. As such it was very attractive to immigrants in much the way that California is now.

About 40 years ago there was a project in Chicago to record the memories of people with Polish ancestors. The archives of the Chicago Polonia Project are stored at the Newberry Library.

You might be interested in listening to this podcast on the topic.