youll get a different opinion from every single person here, but my opinion is this-
1) they dont do it like previous movies because these are an entirely new continuity, entirely new beast in general. the nolan movies are as if the prior four dont exist whatsoever- which in the case of BF and B&R is a good thing. begins and TDK are not prequels, sequels, they are completely new films.
2) nolan is a talented and smart enough director to realize that something new should be achieved by his series. the joker has been done before, but not anywhere near the fashion we are getting him now. a good filmmaker sets out to reinvent the wheel each time- to simply follow the comics exactly would make a stagnant, and ultimately, lesser film. plus, judging by the prologue/virals/teaser/reports, etc, etc...this joker, in my opinion, is truer to the comics than the 89 joker is, personality wise. visually, he's an entirely new beast, but you have to remember that this is the joker as incorporated into nolan's gotham, which is, again, something entirely new.
also, if you look at the interpretation of batman and his villains over the past 60 or so years, theyve all changed so many times and had so many different things done, that this shouldnt come as a shock to anyone. there isnt really a set "way" in the comics. there was a point when the joker existed solely to play silly pranks on batman (which is just...awful), and there are points where he's a disgusting and absolutely vile killer, and nolan is going that route (which is my personal favorite. i like the joker as somebody who's sense of humor is solely for himself, and involves horrible things). whether or not people like it, doesnt matter. thats whats happening and i, for one, think its going to be brilliant.
wow what a rambling post.