This comes back to the problem with old music: music didn't get worse, you just remember the good or memorable songs. At any point since the Billboard charts have been created 70% of them is dross, 20% is mediocre, maybe 10% is good. Everybody remembers the good songs that survive because they are good, and some of the mediocre songs people relate to. Everybody forgets the dross.
But back then, that was what you listened to as well.
(Check out, e.g. the Billboard hot 100 for 1968 (or even just Hot 20): it had Hey Jude at position 1, Sittin' on the Dock of the Bay on 3, and Mrs Robinson on 9, but it also had, let's see... 18 was Grazing in the Grass by Hugh Masekela (which is good but it's a trumpet instrumental), 2 was Love is Blue by Paul Mauriat (a schmaltzy melody, but it's the second hottest song if 1968), and 7 is This Guy's in Love with you by Herb Alpert)
(And you can do that with basically every year. I graduated in 2004, so what do we have there? Usher's Yeah on 1 (I remember that), Usher's Burn on 2 (no clue), Maroon 5 on 4 (this is one of those bands everybody seems to have struck out of their memory), Hey Ya by Outcast on 8, but their The Way you Move on 5 (definitely not a mainstay I would say), Nickelback is 17 (another band everybody pretends never to have listened to), but Twista's Slow Jamz is 16 (who?) )