Well, not everyone made that choice. Some had that choice forced upon them, by slave traders, kidnappers, conscription, war, pogroms, famine, etc. My own family on my dad’s side came during the lesser famine (we think), were from the west of Ireland and likely came out of lack of choices back home. Lots of people did come out of choice, but even then, it was often because they had options here they didn’t have back home and even already had a family network of some kind set up for work.
People would sometimes say the same thing to African Americans during the civil rights era - if the south is so bad, why not just stay. Well, because as American citizens, they should be allowed to live where they want in their own country.
It really does disturb me that people are willing to write off entire sections of the country (going both ways here), ignoring that all parts of the country are often heterogeneous, politically, culturally, and socially. Those of us who live in the south shouldn’t be written off, I don’t think.