With over 75 years of history behind him, Batman is easily comics' most prolific character. Going from his humble roots of fighting gangsters, to the campiness of the 1960s, to the Dark Knight, Batman has an epic catalog of stories that are unlike anyone else in comics history. Batman has grown beyond just being the crown jewel of DC and is now a cultural icon known around the world.
DC knows what marketing gold they have with Batman and have put out an endless number of stories featuring him. Currently, he can be found in 6 separate comics being published. But DC also has a tendency to put their best talent behind Batman, so he consistently has some of the best books out at any given time.
Given that the comics industry continues to struggle to get new readership, DC relies heavily on Batman's name to pique interest, and the writing to keep it. Batman remains the most consistent selling comic book character, and a large part of that has to do with the quality of his comics.
Even as sales waned, he's been one of the most consistently readable comics characters in the 21st century...