The answers below are all factors but he was and average general vs the best in Lee, after all both sides asked him to run their armies. He got bluffed into thinking Lee had twice as many troops than he had, still less the McClellan. McClellan thought it was between 1.5 to 1 and 2 to 1, he wanted the 3 to 1, which the military thinking of time would mean he would win, with the wrong odds he might it would be bloody and he was facing a more experienced and "better" general.
So he waited pulling in more troops which gave away the initiative when (in hindsight) he could have smashed the south and knocked a few years off the war.

There were poorer generals in the war on both sides but he allowed Lee to build a army and then failed to win when he did fight.