I did my PhD in mid-west, and I literally only met 2 American PhD students during my entire PhD (majority was Chinese and Indian students who pursue a PhD as a gateway to a top job). I guess location is a big factor.
It also makes sense why there are less American PhD students in those kind of locations when you look from their perspective. If I were an American, there was no way for me to spend 5+ years of my life in mid-west with 1500K a month (instead, I could just lend to a top company in a good location with $$$$). But for most of the foreign nationals, such an experience is OK to bear with in return for a better life standard.