Here is the answer. Money.
US MSF programs are graded based on placements. Students use these programs to get into a good paying finance job because they didn't do well in UG, didn't realize they wanted to do finance in UG, went to a school without OCR, etc. So they go to an MSF. The best programs have consistent placements and these programs admit fewer international students because those students have issues getting placed.
Years ago some schools decided to simply use the MSF as a way to boost revenues. Most of these MSF programs are just built off the back of MBA programs which have seen a consistent decline in applications. What better way to utilize slack MBA teachers? So they started to admit more and more international students. This causes a vicious cycle because most domestic students don't want to be in a call where the vast majority are international. It impacts placements. So domestic students avoid these programs and they become more and more international.
Tulane is a really good example of this.
30% or so is the right number of international students and they should be drawn from all over. I believe the UIUC MSF program is like 80% Chinese or something (this has probably changed since the trade war). Professors would literally speak Chinese and if you were the one or two domestic kid you definitely felt left out (I've been told this first hand).
Look at Vanderbilt, UT Austin, WUSTL, all have lower international percentages. MIT is the anomaly, but MIT is getting the absolute best international students. Go look at some less prominent MSF programs and you can tell it is about the tuition money and not the outcome.
This is why.