This is, of course, my own personal opinion, but I think it stems from two reasons. The first is that unlike the Greeks, Hannibal's army was facing a heavily armoured, well equipped force of at least equal if not superior quality. There was little if any armour on the field against the Greeks at Marathon, and it showed in the decisive defeat the Persian army suffered there.

Second, the battle itself was far more coniving and successful than the Greeks had been. In Marathon, although the centre was pushed back, the battle was really won (as it was designed to) by the strength of the flanks pushing forward. At Cannae, Hannibal merely needed his flanks to hold while his centre was pushed back, which allowed him to actually complete the envelopment on the whole army, something the Greeks were unable to do.