As opposed to the human emotion.
I had a couple comic book friends back when I was in jr high many years back, and the reason people tend to like peter parker is because he is someone to sympathize with. He doesn't really want to be a superhero, and has a crapton of personal problems. A good bit of people like rooting for the underdog, unlike superman that is nearly fucking invincible.
So, while a guy that can kick ass 100% of the time like superman is pretty cool, it kinda gets old. Not only that, but having a character like spiderman is a different kind of superhero that people that don't like superman can like. The more characters people like, the more sales. :>
TLDR: Peter Parker is a bitch because people like it and it sells more copies.