Expense, perhaps.
Why do we wear seatbelts in cars but not on buses?
I think cost is one factor (which John has already mentioned), but there are possibly a few others:
- The size of a bus usually means the impact from a crash is distributed differently.
- "Compartmentalization" is a safety measure that many buses use instead of seat-belts. It's based on rather old studies, and isn't accepted by all as the best way to protect passengers.
Nevertheless, the theory is that having padded seats which absorb the energy when a person is thrown into the back of one or slammed into the seat they're sitting in.
Unfortunately, they're not that useful for side impacts or when a bus rolls over, which is why more and more buses these days are opting for seat-belts.