https://www.hup.harvard.edu/catalog.php?isbn=9780674976894
Excerpt: "tiny fraction" of low-income black American teenagers who attend private schools "produces about half of low-income black students at Ivy League colleges."
Anecdotally, looking at who attended Ivies where I grew up, this seems to be true.
I think they worry that Black students from actually Black, and often poor, school districts will not jive well with so many silver spoon swallowers. They want people more accustomed to bougie New England & NYC culture.