I think the audience it was aiming at was too specific. I'm an avid movie watcher, yet I'm not a fan of graphic novels at all. So like Sin City or Watchmen or 300 or even Blade I was able to view them from an unbiased movie standpoint and found them enjoyable. Many others, even in the mainstream audience felt the same way because they catered to their needs, violence, gore and boobs.
Despite being a gamer, a geek and have been deeply immersed in "internet culture" over the past 10 years, SPvtW just looked terrible to me and I have no interest in watching it. Perhaps, to quote Movie Bob, I'm "the worst kind of person" simply for not caring about this movie at all but in all honesty I'd say here in Britain you're looking at a 13-19 year old demographic compared to the US's slightly older one.
So, here at least, it's a movie aimed at teenage "geeks" by their own admission, who are the group of people most likely to download or stream instead of actually paying for it. Couple that with it coming out at the same time as the Expendables which will have far more nostalgia for people of my age (I remember being 10 in 1997 virtually everyone had seen First Blood, Commando, Terminator, Die Hard, Predator...so much for age ratings huh?) so it's no surprise that Scott Pilgrim didn't do so well.