Best Songwriters of the 1960s

In David Marsh and James Bernard's The New Book of Rock Lists (Simon & Schuster 1994), there is a list of the 100 Best Songwriters.  The rankings are divided up by decade.  Here's the top 12 for the 1960s.

12. Nickolas Ashford and Valerie Simpson
11. Brian Wilson
10. Pete Townsend
9. Gerry Goffin and Carole King
8. Isaac Hayes and David Porter
7. Jeff Barry and Ellie Greenwich
6. Curtis Mayfield
5. Smokey Robinson
4. Mick Jagger and Keith Richards
3. John Lennon and Paul McCartney
2. Brian Holland, Lamont Dozier, and Eddie Holland
1. Bob Dylan

Lower down on the list, we can find Sly Stone (#14). Paul Simon (#16), Ray Davies (#22), and James Brown (#25).

It's interesting how many collaborators there are on the list.  Neither the 50s list nor the 70s list has so many collaborations, though the 50s has 2 in the top 5 (Lieber & Stoller and Pomus & Shuman).  The 70s list has no collaborations in the top 5 and the 80s/90s list has one (RUN DMC).

Was there something special about the 60s that fostered great collaborations?  Alternatively, were collaborations part of what made the 60s arguably the best decade for music in the 20th century?  Should all of the lists be ignored in their entirety because Sir Mix-a-lot is at #15 on the 80s/90s list?  Did Boy George and Jon Moss benefit from a pro-collaboration bias to make the list (#23 for the 80s/90s)?  Does the list have a bias against bands for whom the main songwriters chose not to list themselves as collaborators (e.g. Bob Mould and Grant Hart aren't on the list)?