Why would you have a day for a 'man' that was 5'7"?
Except that was above average for a man at that time so he wasn't really short at all by previous standards.
I agree with Steven that a big part would probably come from the fact that he lost. Had he won then obviously France, and the rest of the world, would be very different today. However he didn't, and every victory that he ever won was rendered virtually irrelevant. Besides, for me at least, Napoleon would be a symbol of the revolution considering that is where he rose to prominence, which is celebrated on Bastille day.