the late 60s had so much going on. monumental work from all of the following: Miles Davis, Coltrane, Dylan, Beatles (basically their entire creative peak), BB King, The Beach Boys, The Who, Jimi Hendrix (entire career), Aretha Franklin, The Rolling Stones, Johnny Cash, James Brown, Buddy Guy, Loretta Lynn, Marvin Gaye, etc. most of whom were at their absolute creative peak. then you had the garage rock scene led by The Sonics, the growth of funk, the birth of metal, the birth of jazz fusion, the folk/blues revival with important albums from Skip James and Son House, a huge period for electric blues (and heavy blues), several awesome Motown singles, etc.
i dont know the exact reasons for it. maybe the social climate motivated the music being made, maybe it was just a perfect storm of different genres competing with each other and merging, maybe it was technological advances. i dont know. but something special was going on. and i dont think its just cultural nostalgia either.
Ahem what? The birth of metal comes in 1970.