The main reason? They're rare. In the trading circles, they're one of the single most valuable items, and can be traded for just about any single item, no matter how valuable, and often multiples of anything else.
Why? They were a one-time promotion, only given out to players who logged into TF2 from a Mac during a set time period, max of one per account, during a time when the game didn't have quite the same population it does now (I think it was before the game went free to play, so the population was close to its lowest point).