Because when he assumes the cape and cowl, he becomes Batman. The voice is part of the persona.
You have to remember that he still newly Batman, even in The Dark Knight. Most Batman stories take place when Bruce is around 40.
So... he hasn\'t mastered using a fake voice yet.

As for this movie, it\'s amazing (I\'ve now seen it twice) but there were just waaay too many young children at each showing. Such a dark movie.