Because of the worthless skits/filler tracks.  Yes I would give it six stars if that were allowed and there were no silly skits and filler.  This, to me, is one of the greatest albums ever made, by the greatest band ever...to me.  I don't like Tool because they are "smart", "witty" or "pretentious" or whatever the people who don't like them want to label the band.  I just like them because the music is candy for my ears.