The tires are a huge factor. Those big knobby tires suck up speed like nothing else because of the extra rolling resistance. There are plenty of other factors, too: your suspension (especially the rear) saps pedaling power, your mountain bike is much heavier than a road bike, and the mountain bike puts you in a more upright position, which will really hurt you in a headwind or if you're going to push speeds up over 20 mph.
You may in fact have a very good mountain bike, but short of a pair of slicks and lockouts on the suspension, it will always be out of its element on pavement.
Edit: road bikes are also geared higher (=faster) than mountain bikes, but unless you're spinning out on your mtb, that's not really an issue.
Last edited by DaylightsDrknss; 11-09-08 at 01:37 AM.