• The Internet Sucks: Another proven case that the internet is full of losers that don't actually go outside of the house. The social networking behind the movies marketing was excellent including the ability to create an avatar of yourself. It was a twitter topic trend going into the weekend. So what? People didn't see the movie. Internet popularity doesn't equal summer blockbuster. Otherwise we would have a giggling baby movie.
• Cera being Cera: Michael Cera has had a hard time overcoming being himself in movies recently with his sort of impish nerdy character translating and typecasting the movies he is in. Ironically, the character he plays in this movie is the most different mainly because he is sort of a jerk. I think it is to the point that people look at his face and they say, "oh that kid" and move on. Someone needs a makeover and possible should go back to TV.
• Scott Pilgrim TV: The story takes 7 books and concentrates them into one movie, very hard to do. The story would have suited as a live action or animated TV series much better to develop stories and take it's time. The movie is a fun ride but in the long term franchise, would have made sense. Even doing it as a Adult Swim cartoon first would have helped it down the road to make it a live action movie.
• People like Explosions: The Expendables has it, tons of it and the action in SPvW is great but wasn't really marketed that way. Too bad, was great kung fu.
• Hipsters don't watch new movies: Period, they just watch classics like PeeWeeHurman or something.

I'd have to attribute it largely to Michael Cera as well. A lot of people I know who would otherwise like and be interested in seeing the movie immediately changed their minds when they found out about Michael Cera.  
 
I wish I could have seen it opening weekend, but it's not out in my local theater yet. Hopefully it does well enough for them, I really enjoyed the series.

I hate Cera and when I first saw he was the lead actor, I wasnt gonna see it. But man am I glad i did.

I heard that it was getting good reviews

It got better reviews than the Expendables. And I really liked it...

I think is was the fact it went against The Expendables and Eat Pray Love, both movies has an audience. The Expendables is targeted for the male audience, and Eat Pray Love was targeted for the female audience. Who was Scott Pilgrim target for? It's not a movie your mother is going to want to go see. 
 
It was basically put in a bad situation, if it was brought out this week where it's only competition is Vampire Sucks and The Lottery Ticket, it probably would have done better. 
 
Also 90% of the human population are hypocrites, we deny it but it's true. People whine all the time about "Hollywood is out of ideas!!! They aren't creative or original!!" but when an original and creative movie comes along that has been receiving almost nothing but praise from all the critics, they would rather ignore it and go see a typical 80's style action movie or another bad run of the mill cliche romance movie instead. 
 
However I'm willing to bet money that this movie makes it's budget back in DVD sales and becomes a cult classic, so it's all good. 

Just wait. A lot of people, especially from its target audience, don't go to movie theaters anymore.