The Doors are one of those bands that was perfect for its era, important and influential -- but just doesn't hold up over time. The music feels very dated and insignificant today, the absence of bass that Jack Black trumpeted in the High Fidelity movie is revealed as a weakness, and much of the "poetry" spouted by Jim Morrison turned out to be a bunch of pretentious hooey.
Cream is likewise dated. Three unbelievably talented musicians, probably the greatest power trio ever assembled (with all respect to Geddy, Alex and Neil), but we saw how their album went down in flames in this poll. When Disraeli Gears was released, it was probably mind-blowing. People sprayed "Clapton is God" graffiti all over London, apparently. But now, it kind of seems lame. When Cream stuck to the blues, they were really good, and that stuff holds up. But their psychedelic '60s stuff like "Tales of Brave Ulysses" just sounds like crap now.
Contrast these bands to the Beatles and Stones albums that hold up great today. That music is pure and timeless and blows most if not all of today's acts -- not just the bubblegum pop, but the critically acclaimed bands like Tame Impala and Arcade Fire -- out of the water. That's also why we still listen to Iron Maiden but not Cinderella or Poison or Whitesnake (at least, not as much). The latter bands' music is firmly rooted in the hair-metal/glam era, but Maiden's music is (mostly) timeless.