It's funny to me how wrong all of the answers of this thread are: These programs are too broad. / The talent is abroad. / This is a ticket for a Visa.

Those may all be more or less true, but the broader issue of why these programs are mostly populated by foreigners is that they're too expensive (for master degrees) and vastly underpaid compared to market rates for PhD degrees. (Consider the strike at UC Santa Cruz by PhD students for a wage that would allow them to cover their living expenses without having to commute hours to campus.)

US students, which most likely have already contracted debt to finance their undergraduate degree will simply go get a job that both pays better and may boost their carreers more, and may come with more social status, given the current wave of anti-academic sentiment.