Low admittance rate is a plus for most college rankings so institutions are incentivized to engage in efforts to increase the size at the top of the funnel.
Personally I think absolute number of admitted students is a more useful metric, since you can't really control how many people apply, and your capacity is more or less fixed.
From the graph in the article, that is also in decline, but not as sharp (except the outlier of this year, which is caused by other factors).
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.
Low admittance rate is a plus for most college rankings so institutions are incentivized to engage in efforts to increase the size at the top of the funnel.
Disclaimer: Princeton alumnus.