I recently picked up a copy of one of Joel Whitburn's Billboard charts books (specifically The Billboard Book of Top 40 Hits, 7th edition), which I find endlessly fascinating. The one I have lists as an appendix the biggest hit singles of the rock era up to 2000 (I got this at a used bookstore), and there is something that I find inexplicable about this. The top nine singles of all time by Whitburn's reckoning all date from the decade of the 1990s. Then there's "Don't Be Cruel" (by Elvis, not by Bobby Brown or Cheap Trick), followed by two more 1990s hits. Then those 1990s hits are followed by three more 1950s hits (none of them by Elvis), such that every single one of the Top Fifteen hits of all time dates from either the 1990s or the 1950s. Finally breaking the string is Olivia Newton-John's "Physical."
I've been trying to figure out why that would be; there must be something structural in the pop charts and the way people buy music that would have made the 1990s (and I'm guessing the 2000s) so dominant in this category. The only thing I can think of is that whereas every song released in the 1960s and 1970s made an effort to become a pop hit, many successful modern-day acts make no effort to dent the pop charts, leaving that field for pure pop acts like Mariah Carey. But I don't really know.
Consider this: "I Want to Hold Your Hand" was the biggest hit of 1964, a Number One single for an impressive seven straight weeks. But "I'll Make Love to You," by Boyz II Men, spent as many weeks at Number One in 1994 as "I Want to Hold Your Hand" spent in the entire Top Forty.