Wisconsin and Michigan have coasties. Iowa and to a lesser extent, IU, do not

...and was false when I attended IU in the mid-1990s. I can't find a state-by-state breakdown of IU's student body, but according to promotional materials from when IU played in the Pinstripe Bowl in 2015, students from New York and New Jersey alone account for ten percent of IU-Bloomington's enrollment. It's also worth noting that a non-trivial percentage of in-state students are commuters or non-traditional students who aren't part of campus social life, so living on or near campus it feels more geographically diverse than the numbers would suggest. Of course, it doesn't compare to ND's geographical diversity, but Big Ten schools aren't the provincial backwaters you imagine. (None of this is to disagree with the main point that ND shouldn't join the Big Ten).

My own experience from 20 years ago in a decent size Long Island public school I had 4 classmates go to IU that I can remember, one buddy go to Wisconsin and maybe 12 to 15 go to Michigan,

There’s a big Chicago contingent, certainly, but they have a large NY/NJ population, especially in the Greek system and in the business school. Big Ohio contingent there as well.

Nonetheless, all of them are regional schools. I’d bet none have more than 10% from more than a state away except maybe Michigan.

The Greek system at IU has become a draw

Contrast this with nd which has CA, Ny, NJ and Florida in their top 7 or 8