Because he deserves it.
He deserved it when he was 13 years old, playing a young J.G. Ballard in Spielberg's Empire of the Sun, which Art Attack saw when we were 15. It made me blubber in tears then, and it still gets me when young Jamie (Bale) confesses to the British doctor, "I can't remember what my parents look like," after he ecstatically watches American P-51 Mustangs--"Cadillac of the Sky!"--bomb the airstrip of the WWII Japanese internment camp where he's been imprisoned.
The 1987 film is one of my favorites of all time (and certainly underrated for Spielberg).
Although I distinctly remember John Malkovich being nominated for Best Supporting Actor for his role as the charming rogue Basie (all internet searches dispute this--perhaps it's just what should've been), no nomination was given to Bale for Best Actor, even though it wasn't unprecedented for a young actor to be nominated--Tatum O'Neal won the 1973 Best Supporting Actress Oscar for Paper Moon. Bale wouldn't have had a chance against the other nominees, which included Jack Nicholson, Robin Williams, William Hurt and Michael Douglas (the winner, for Wall Street), but it doesn't excuse the fact that he hasn't been nominated for a major award until now.
This Sunday's Golden Globe Awards marks Bale's first GG nomination (for The Fighter). He has yet to be nominated for an Oscar.
We present some examples of why Bale is glaringly the most underrated actor of his generation.