What is the reason for this or is it just like that.
I believe many schools limit international school students to 10-15% of student body. Getting off the waitlist may depend on someone from the admitted international students to decline their offer. 
So, how often do you think international student spots just end up going to someone from the same country or area? Like, if an American drops out, could a European student with a similar profile (not an athlete, non-FA, plays music, etc.) snag that spot? Or is it pretty much guaranteed to go to another American?
Schools have to do a bit of paperwork at their end to support international students’ visas and often need to support them in other ways ( i.e., through shorter breaks), so most pay attention to the number of foreign students enrolled. While every school has its own way of managing this, it makes international boarders less "interchangeable " with domestic students. This is more art than science.
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Also some schools limit the number of students from any one specific country too. So if you’re an applicant from a country that sends a lot of kids to a certain school, you may need an admitted student from your own country to decline to get a spot.