Before the late 60s, the Ivies were basically glorified prep schools for rich men's sons earning gentlemen's Cs. The only people who needed to demonstrate real academic brilliance were the ones who required financial aid -- the poor and minority students. Donald Trump is another person who benefited from the earlier, looser standards for rich white guys, when he got into Penn's Wharton School.
All that changed for a number of reasons but partly as a result of the admission of women at universities like Yale and Princeton. The women were being held to higher admissions standards than the men, and there were also, suddenly, fewer places for male applicants, so standards as a whole rose. By the mid-70s, these institutions had changed enormously and become very selective. That process has continued.
I attended Yale myself and later earned my PhD at Penn, so I'm quite familiar with the history. The Yale Alumni Magazine published an interesting story on the subject back in the 1990s.