<p>One factor is competition from flagship public schools in the various regions. In New England, New York, Pennsylvania, and New Jersey there are no publics with any prestige, and IMO only Penn State has any esprit at all.
Every state in the South has a "destination" public or two, typically with school pride, and often with the majority of the best students in their states. All of these schools have a public function and tax payer support which pushes them to expand the size of their student bodies rather than become increasingly selective. I'm including Texas, Texas A&M and Oklahoma here. This does not work to improve their "rankings".
In contrast the privates work hard to improve their selectivity with the approval of the donating alums, so long as a generous number of spaces are reserved for their offspring!
And then, of course, there is shear wealth.</p>