The Kings Speech over Inception a couple of years ago was a joke. The Academy hates anything action or sci-fi and has an orgasm over Colin Firth and anything he's in. Inception is easily one of the best movies in the last 20 years
Because most action/blockbuster movies lack real quality acting, cinematography, direction, storyline, etc.
Oscar's aren't about popularity or gross numbers; there's still at least some level of integrity to the quality of movie they award.
I do think there should be a category for best action/adventure and for best comedy though.
As for best picture.....well, watch a movie like Moonrise Kingdom or Silver Linings Playbook and there's some serious acting and directing going on in there that something like The Avengers just can't compete with. It's artistic integrity.