Kick-Ass was a deconstruction of the Superhero genre the same way Zombieland tackled the zombie genre. Both were highly rated, and (I thought) pretty well promoted, and both made within 25mil of eachother, Zombieland barely breaking the 100m mark and Kick-Ass, as we know, currently around the 75m mark.
So it's a good question, but I think it's what others have pointed out - these are niche films. Maybe it's because I'm in that niche, I can't pull my head out of the box long enough to see how anyone could NOT want to watch these awesome films.
I think it may also have something to do with our current collective taste in entertainment. Here in the US, we're fighting 2 wars, the economy is, well, you know, and people want to be transported, not reminded of the bleakness and darkness of the current state of affairs. That's why all those Iraq war movies have either flopped or barely broken even. Remember during the Vietnam and post-Vietnam era, they hadn't figured this out yet, and the market was filled with dark depressing films that reflected the mood of the times - disaster movies and gritty cop films. Then some guy called George Lucas figured out this was not what the people wanted to see and made the biggest hit in the world at the time.
I think somewhere in their minds most people don't want to shell out $10 to see raw bloody violence which both Kick Ass and Zombieland had in spades. Think about it: has any movie so far this year that featured loads of people getting killed been a roaring success? The Losers lost, and prior to that Ninja Assassin made less than 20m more than its budget. Honestly, I fear for both The Expendables and The A-Team.
Yes, loads of people are getting killed in Iron Man 2 and Avatar, but it's for a righteous cause - an upstanding superhero and defenders of their beautiful homeworld, respectively. And then there's also tremendous hype surrounding these films as well.
Basically what I'm saying is these days people will only see a film with lots of gruesome deaths if A) the good guy is the very embodiment of the archetypal hero and B) The bad guys are really, really evil. The gap between good and bad needs to be exaggeratedly massive (which, in my opinion can sometimes prove to be so extreme as to be laughable). Basically, films where both the good and the bad are are not clearly distinguished, that is, it takes place in some kind of gray area...they will not be extremely successful.
Why? I don't really know...my guess would be that the movie-going public doesn't want to have to think about a film - a movie with a clear-cut protagonist is preferable to one with an antihero and a bunch of questions about ethics and morality.
However, this type of genre seems to flourish on the small screen: The Shield, Sons of Anarchy, Justified, The Bridge.
(originally posted to imdb message boards - by me - 5/11/10)