Partially 'cause of Heath Ledger, perhaps.

Yeah, that and the fact that the first one was such a success with critics and audiences.
The movie was generating a lot of buzz even without Heath Ledger's death.
I, for one, am SOOOO excited!

im soooooooooooooooooooooo gonna watch it

(From Wikipedia)
After its first weekend theatrical run, The Dark Knight set new records as the biggest-ever opener with $158,411,483 and biggest-ever single-day grosser with $66 million held on Friday, July 18, 2008, surpassing Spider-man 3 with its $59.8 million Friday and $151 million weekend.[111] The picture also owns new records of biggest midnight release ($18.5 million on Friday), biggest weekend on IMAX screens ($6.2 million), top single-day take on a Friday ($67.9 million), and most released screens (4,366).[citation needed]