Because they're ultimately childish. Now hey, sometimes that's fun, but I expect better from Oscar nominees. Not that Oscar is completely pure, after all they did nominate Depp for POTC(a bad joke, IMO).
Yes, and the people nominated for ten thousand biopics and period films, churning out the same performances every time in movies which are only vaguely dissimilar from eachother is so very "pure". The Oscars became irrelevant a long time ago, and will continue to be irrelevant unless they stop being so pretentious. Ledger's given the best performance of the decade, let alone the year and if he's not nominated the awards will lose all credibility.
To a lesser degree, Downey Jr has given a great performance in Iron Man and I wouldn't exactly be amazed by him getting a nod of some kind. Will that happen? No. TDK is the best movie of the year. It deserves the statue, not a nomination. Will it get it? Nooo. Some run of the mill period piece/biopic will.