Those of you who follow my twitter (d20sapphire, unsurprisingly) may know that while running RPG Chicago I have lobbied the idea of a One Shot Night in multiple locations in the area. I know a lot of people love being in long standing campaigns. You guys know that I love running one. But the one-shot... it's a glory all on it's own.
The thing about a campaign is that everyone starts it knowing that it's an investment. With each session you have to give enough to keep the players involved, but not every session has to be on point. In fact, it's almost impossible to keep it on point, because every player is going to have goals that diverge from your main plot. As a DM you become half story-teller, half manager. It's still fun, but it's not the same as a one shot.
The one shot is a one-time chance to prove to the players that this story is awesome. It is a one time chance to give them something sensational, and it has to hit immediately. Slow story is not an option.
Players love it too. For once they have a character that they don't have to fear about losing, they can just be reckless and take bigger chances. It's time to shine, because this character only has once.
The glory of the one shot is exactly that: you have one shot to make an impression. And it makes the difference in how people play.