It may not have peaked in '71, but there's no doubt that it fell flat not too long afterwards.
The '76 list itself makes a good point - that many albums, of quality, in a year? I personally couldn't fill a list that long from the last 20, even 30!
Good list of albums released across that whole year here -
https://en.wikipedia.org/wiki/1976_in_music#Albums_released
Some super soul in there too....and Jaco's debut! Had I not been 2 years old, I would have been seriously broke every month!
I don't think it's just rock. Take most years up to the early 80's, and you'll find a treasure of rock, soul, jazz.
Maybe things come in waves. There was some great American 'alternative' stuff that came up in the 80's, which led in to the 90's, but since then, most rock music has left me cold.