The hell of it is that Nolan could have done it, and set up Bruce Wayne's knowledge of martial arts to do that in Begins. He just chose to go with Keysi because those guys were promoting the hell out of their garbage in Hollywood at the time. Come the third installment and those guys aren't even doing their own thing anymore. I'm in no way convinced that a guy who had such a vast knowledge of East Asian martial arts, as Bruce is shown to have when he faces off with Ducard/Ra's after getting to the League of Shadows temple, would ever come up with something like Keysi as his own style.
Plus, there's something to be said for personal fighting style/flavor and creating one's own martial art. Bruce doesn't strike me as the kind of guy to just create his own martial art, he's there for utility and his vast knowledge means he's got a huge arsenal to work with. To create his own martial art means he'd be paring things down to a select few techniques from everything he's learned as a focus and Bruce Wayne is just so dedicated to mastering as many techniques to use in any possible situation that I don't think he'd really ever spend much time saying to himself "well, this martial art that I learned has a whole bunch of stuff that is useless except for these few things" I think he's actually the sort to approach his martial arts with an open mind and the thought that whatever he learns could potentially have utility given the kinds of situations he's going to throw himself into. It's just fortunate that there's so much overlap in martial arts that, and this is actually true, once he's mastered one other styles come much easier to him. And the interplay of all those different styles means that kung fu A becomes informed by Japanese style B and both by Malay style C etc etc.