I think one of the reasons for this is that we have so many top notch boarding schools in New England. I live in Massachusetts and there are fifteen traditional private high schools with boarding programs within an hour of my house, plus a few additional junior boarding schools and non-traditional BS. 20 of the top 34 BS’s on Boarding School Review are in New England. Another 5 are in California, leaving only 9 for the rest of the country. The boarding school tradition is just deeper in the Northeast than the rest of the country.
Of my daughter’s class at a private K-8 about 20 ended up at schools with boarding, but they were spread among more than a dozen schools. Lots of different kinds of kids who could all find a match locally. Only one went outside New England, and that was to a specialized school.
I do see SAS and Episcopal as popular schools for NE kids going outside the region. Thacher, Cate, and Lawrenceville are also on a lot of NE families’ lists for kids who want to look a little farther afield.