Only because he is dead. If he was still alive there is no way his performance would get an Oscar. It is the definition of average.
Do you like TDK then of course you would say that, hell there are films I don't like but can understand the quality of acting in it. Theres too much of a prejudice about what genre of movies should win Oscars and it needs to change and lets not forget, there was PLENTY of Oscar talks before his death. If the does not win then I will never consider the Oscars a worthy Academy ever again.
His performance was fucking amazing and to say otherwise shows you know nothing about acting.
Also reading some previous posts... Heath was not the lead actor, his name was 2nd on the bill, screentime also and not too mention the film being a BATMAN movie means he cannot be nominated for Best Actor. There is nothing wrong with the best supporting Actor award. For Best actor my money is on the actors from the Nixon or MILK movie to claim that honor.