2. The Audience Would Want Him To Win
This is somewhat of a meta-answer, playing on the audience and reactions as opposed to the characters themselves, but give it some thought. If Superman, and Batman were to brawl on the big screen, who would we want to win? and in the same vain, how would the fight reflect that result. If Superman were to win, one has to think, would there be any real fight, or exchange of blows? In any ordinary encounter, Batman literally can't hurt Superman without the help of outside tools, and he certainly doesn't have anyway of taking advantage of Superman's weakness to the arcane. So how would this fight go? Superman punches Batman once, and breaks his rib-cage? Abruptly melts his face with heat vision, or throws him into space? Sounds pretty anti-climatic, and fatal to me. But for an entertaining spectacle, Batman would probably have to claim victory. Via the use of his surroundings, tools, outside factors and strategy. He's always the underdog if the fight goes to blows, he's just a man. For the sake of the audience, we'd want Batman to win, we'd want to the underdog to overcome the odds and take down the super-powered gloryhog, because the alternative sounds pretty dull and unfeasible. Not to mention the fact Batman is generally more popular among fans, primarily because of the incredibly successful Dark Knight Trilogy.