One of the big factors contributing to the smaller number of internationals in English PhD programs within the U.S., I would tentatively assume, is the fact that, in the wide World, there exists a gap between "native or near-native speakers of English" and "non-native speakers of English (or non-speakers)", and the former group forms a far larger percentage of the U.S. population than of the non-U.S. international population (in which non-U.S. but English-speaking countries are a minority). And members of the former group, of course, are more inclined to be interested in making a living out of, and also to excel at, English as a discipline (which differs innately from other fields in which the use of English would be just a tool in the education process.) So I'd say international students applying for English PhDs are both less common in the first place and indeed "weaker" on average, hence a bigger domestic applicant pool and more domestic acceptances. Forgive me if I sometimes state the obvious; this was me trying to take your question as seriously as possible.