just an FYI, I too had written a diatribe defending the 80s...but that's not what the OP was actually after - he means why did established acts from the 60s and 70s do their weakest work in the 80s. Thread could have a better title I think, maybe he should ask for an edit.
ah, i think it had to do with the direction music was going and how different it was from what those artists were.
The 80s music was driven a lot by technology. you had new recording techniques, sampling, synthesizers, computer generated sounds, etc.
Those became extremely popular in pop music when applied over dance/pop based drum beats and gave birth to New Wave and the 80s sound in general. The younger acts that were driving that style became the new stars. Michael Jackson, Prince, Madonna, etc.
In Rock, you had a similar shift. The tempo of the songs sped up and more emphasis was put on excess both visually and musically. Which gave birth to hair metal and every band having wank fest solos in every song, and songs became shorter to get on radio
The problem is both of these factors were completely different from what popular acts of the 60s and 70s were used to. Music in those decades were slower, more blues based, long intricate songs.
When acts like the Rolling Stones, KISS or The Who tried to adopt the "new" sound that was popular, they couldn't do it because it wasnt a part of who they were as musicians. So instead it came across as a bunch of dads trying to be hip.
Basically instead of sticking to what they were great at, they tried to follow the trend and failed miserably.