Billy Buttsex
Yeah, how could any studio exec POSSIBLY say, "you know what? That was hilarious and way better than anything else on TV, but I think we're gonna cancel it for some show about gay sex."

StanleyPain
Well, to be fair, Mr. Show wasn't canceled. Bob and David just decided to stop because they felt they had given it their best and that a 5th season would push it too far. They thought that the 4th season, in it's entirety, was so good by their own standards that they couldn't top it. But, that said, HBO did not give much love to Mr. Show. In fact, the final season was given the worst time-slot on the entire channel, snuggled in with all the late-night shit like "Taxicab Confessions" and "Real Hidden Cameras".

IrishWhiskey
Oh Billy, did the gays ruin something for you again?

Aelric
HBO did give a few concessions to Mr. Show. I do remember lots of ads for season 4 and an all day marathon for season 3 a few days before season 4's premiere. But yeah, after the premiere, HBO made it disappear.