Well, I think to a degree it's about the way the human brain tends to organize itself, compounded by media, historians etc. Nobody's talking about 1989 or 1999 as breakthroughs, though arguably the early 1990's with Nirvana etc was truly revelatory.

I thought we might get some picks for 1971, with, off the top of my head, Led Zep IV, Who's Next, Sticky Fingers, Pearl, multiple others, or 1970, with All Things Must Pass, Derek and the Dominoes, Cosmos Factory, etc.