The Academy likes to make people think they "get it" and will nominate something atypical like The Avengers. Plus, people have been predicting it getting a nod because rumor is they are taking into affect popularity and impact of this movie.
Long story short, typical bullshit politics but in our favor this time.
Oscars are such a crock of manure. They feel since they gave awards to The Rings movies that it shows they're colorblind to genre, but the fact TDK didn't win proves the glass ceiling is as ever present as always, and the high horse its just as high too.