IMO, the factors driving this are:
1) US News rankings are all-important, and the percentage of applicants accepted is a major factor in how they rate schools. The more kids a school can get to apply, the lower their overall acceptance rate. This is one reason some schools have eliminated application fees and essays--kids can just send their Common App and they are part of the pool of applicants that becomes the denominator of the acceptance rate calculation. I think this is why schools like Vanderbilt, the Ivys, etc. send many of their mailings--they certainly don't need to raise their profiles to attract enough good students to fill their classes.
2) The Common App makes it easier to apply to multiple schools, so more kids do so. The fact that acceptance rates have fallen so much also makes it necessary to cast a wider net, which creates a feedback loop: more kids applying to more schools lowers acceptance rates and makes it necessary to apply to more schools which further lowers acceptance rates.
3) A factor I don't think has been mentioned in this thread yet (?): Birth rates declined sharply during the Great Recession and kids born during those years will be applying to college starting around 2026. Colleges know this and are planning for the number of applicants to decline significantly. Many schools outside the top tier will have to market themselves to survive. And with costs rising at 2X or more the rate of inflation for many years, more and more kids will decide to attend community college or take some other path, which will further shrink the applicant pool. Google "college enrollment crash."