That's because Miami is used to contending for national titles and playing in big time bowl games, whereas the Peach Bowl was a big step up for LSU. Miami didn't care about the game.
What I hear is that Miami doesn't travel well at all. Something about the majority of their alumn not being from Florida. I was told the amount of fans they brought to the Peach was pretty typical for their fan base.
Miami fans in general haven't been very supportive. Anyone remember when the Dolphins-Colts playoff game in '00 (played right before the Saints-Rams game) wasn't even sold out in Miami? How do you not sellout a playoff game? If we get a home game, we'll sell it out in a matter of hours.