1. He Already Has
Is there any other reason better than, because they've already fought? Maybe not in the 'standard canon' if you want to nitpick about the context, but in Mark Millar's highly acclaimed The Dark Knight Returns originally published in the 80's, with a 2-part animated feature in 2012, Batman defeats Kal-El. The two are driven to fists as Batman becomes the head of the mutant gang after defeating their leader. The city rejects him, leaving now shamed political lapdog Superman to deal with him. He suffers a fairly crushing defeat. How? With the use of a hardened combat battle suit, and a weakening dose of Kryptonite gas Batman had Superman on the ropes. In many ways this ties together many of the other reasons on the list, preparation, technology, the existence of Kryptonite. Admittedly this happens in a rather 'altered' universe and isn't to be taken strictly as history. Through his triumph we must ask ourselves, would Batman conquer Superman in the same way? Or would outside factors, intervene?