I remember reading somewhere (possibly XKCD) that native English speakers do have a preference for two-syllable words, and an even more marked one for those with a certain other pattern, something in the stresses or something. The whole Ninja/Pirate/Monkey/Robot/Zombie/Viking thing fit, as I recall.

"Batman", as far as I can tell, fits that pattern, too.

Of course, the fact that his whole character design is ridiculously archetypal certainly didn't hurt. But I think there may well be something phonetic there, too.