Not sure if I would call it bias exactly. If you just think about it for a bit you realize that it just shows that most people here have grown a bit weary of him since he has been handled pretty badly in X-Men comics in recent years. It's maybe different for Avengers readers since Wolverine hasn't appeared in all Avengers books written by 2-4 different writers simultaneously, as far as I can tell. There he has been mostly confined to one book/team/writer's vision of him at one time. On the other hand he has been present in half of the x-books for almost a decade now and usually all of those x-books were written by different writers so his character has suffered from the inconsistent characterization and readers of the X-Men suffer from Wolverine exhaustion. I used to love Wolvie when I got into comics and he was my favorite for a long time but I've grown tired of him for exactly these reasons I mentioned.
Aside from that it probably also depends on whether most people answering the poll are fans of Marvel through comics etc. or only movies/cartoons/games and other stuff with having little familiarity with the actual comics. And there is a difference because even thought I really don't like any Avengers in comics these days I still feel that movie Avengers can be very likeable.
So it's kind of same if you asked people whether they preferred Spock or Peter Parker Spidey. People who read the Spidey comics might answer either one but those who haven't read the Spidey comics wouldn't even be familiar with Spock since he hasn't appeared in movies/cartoons/games/whatever.
And btw, what exactly makes you think that he doesn't win or place really well? He seems to be on the lists of many posters even if he isn't number one on most of them so he'll likely still place very high because let's face it, we all probably know which characters will be in the top 10 for certain.