Because, not only does he completely bastardise my religion (Well, Ghost Rider kinda does too, but not nearly to the degree Spawn does), but he's also a piece of trash character. He was a CIA black ops badass, but was double-crossed by his boss, so in order to return to Earth and see his wife again, he got rooked into a Faustian deal with a demon called Malebolgia, who transformed him into a poorly designed clusterfuck of spikes and chains. Seriously, he looks like what would happen if one of Spider-Man's enemies punched him into the coat check room of the local Goth bar.
I can still barely follow or understand that much about him, and that's just his origin story.
He is EXTREMELY overrated. Probably because he's nothing more than a walking symbol of empty, juvenile fantasy-fulfilment; the embodiment of adolescent predilections for gratuitous splash pages full of boobs, blood, and blades. Spawn is a character that has done absolutely NOTHING to deserve his iconic status in the comics world.
Seriously, think about it. In over 200 issues of meandering nonsense, can you name a single memorable or critically acclaimed Spawn story? Don't try to answer that, you'll just hurt yourself.
Sorry, didn't mean to go into detail there, I just REALLY hate Spawn.
Anyway, your vote?