Because it's just a damn good movie.
I'm not going to be able to write much of a review here without spoilers, so I'll keep it brief. I've heard people saying this is the best comic book movie ever, or the best superhero movie ever... and I disagree with both of those (those are a tossup between "Sin City", and "300"; and "Iron Man" respectively) but only just off the mark in either case.
The important thing to note though, is that whether you like comic movies and superheros, or hate them; if you like good movies (as opposed to "fine cinema") you're going to like Dark Knight.
Note perfect performances; the only problem being, there are so many great characters there, that there isn't enough screen time for any of them (including the two leads by the way)... In fact I'd say the real star of the pic is Gery Oldman as Gordon, rather than Christian Bale.
There were only two plot devices, one that showed up twice, the other just once; that made me hit my suspensions of disbelief wall... which for a superhero movie is pretty damn good by the by. One was simply overly and unnecessarily complex and elaborate, and unnecessary to use when they did (being cool for coolness sake is somtimes OK; this time it didn't work); and the other made me yell out in the theater "Damn... physics just got made batmans bitch".
There are some DAMN good twists... not exactly completely unpredictable (especially if you know the comic), but very well executed.
Oh and a semi-almost-kinda-littlebitofa spoiler here for just a second. For those of you upset about a particular twist... Remember, there was no body, and the joker is twisted enough to have done any sort of thing... so don't count on what you saw being what you saw.
So, seriously, just go see it. At two and a half hours, it was still too short. I could've at for another half hour and not cared... maybe another hour... it was that good.