A young friend of mine asked me if I was interested in going to a mutual friend's house one evening to play Munchkin. There were several reasons I could not, but one was "it's too silly". Munchkin is a deliberately silly game. This is amusing for a little while, but after that it just gets in the way.
Yet, when I was playtesting one of my zombie games I said to the players, "it's a silly zombie game after all". But the silliness is of a different kind, and I asked myself what made the difference.
What it amounts to is that zombies are silly, but they can be played "straight". Zombie movies sometimes play them straight, but the strongest example I can think of is Max Brooks' book Zombie Survival Guide, a relentlessly straight (yet reasonably humorous) treatment of the possibility of a zombie apocalypse. (I imagine his "World War Z" is also straight, haven't read it, but I see it is being made into a movie...)
In other words, you can pretend that zombies exist and play it "for real". The silly humor in Munchkin just doesn't translate to even a pretend reality. Not one I can believe in, anyway.
Obviously, other people don't have that point of view, as Munchkin is very popular.