There are a few reasons:
1) In an accident, the larger the ratio of your vechiles weight to the other vehicles weight, the less damage you will have. There isn't much that a bus can hit that weighs more. Thus in normal collisions with cars, signs, SUVs, etc... the damage done to a bus is minor. Only if a bus hits a brick wall or a semi will there be a need for restraints.
2) Seat belts are primarilly meant to keep you in your seat (flying out of a car has the highest chance of being killed). The high seat backs in a bus prevent most people from flying out of the bus. Even if you fly over the seat in front of you, the shear bus size makes flying out of the bus highly improbable (you will get some bumps and bruises, maybe a broken bone or two - but you will still be alive).
3) Most serious bus accidents occur when a bus overturns (see #1 for the reason that the rest of the accidents are minor). If a bus of 30 small children overturns how will a driver have enough time to unbuckle 30 kids before a possible fire burns them? Note: on city busses that I ride all the time it takes 3 minutes for a driver to buckle/unbuckle one handicapped rider. Imagine 3 minutes times 30 kids in a fire...
4) No federal law.
5) Too much hassile in non emergencies. Imagine being a driver that has to enforce a seatbelt law. Every 30 seconds you'd have to stop the bus to buckle in another unruly child... Sure the driver could ignore the law - but then he/she would risk heavy fines.
Some local places are starting to make seatbelts manditory in busses. This is a trend that will likely do little to save lives in my opinion.