Because no other talking head on television has anything close to his game.
This is about as mature and well-informed a discussion on the topic of gay marriage I've seen, but by the end—and I know I am biased—Stewart just takes Huckabee apart. Huckabee tries the slippery slope argument, the "gay as a choice" argument, procreation, childrearing, and everything he has up his sleeve...by the end of the discussion he's reduced to defending an entry in the dictionary over human rights.
Game. Set. Match.