[csa]

Absolute number admitted is probably decreasing because the admissions committee is more effective at offering admissions to people who are more likely to accept the offer. There are a number of ways to do this.

Without looking, I would guess that that the entering classes of Princeton and other elite schools largely stay fairly steady over time for a variety of reasons.

As for the number of people who apply, this can be increased via outreach programs, marketing, etc.