It's cost/benefit analysis. Same reason buildings in Los Angeles aren't built to withstand 9.0 quakes. The drawbacks would outweigh the benefits.

Seat belts are intended to do two things: keep you in the car, and from slamming into the hard things all around you at high velocity. It's rare that either one of these scenarios will happen with a school bus, partly due to the mass of the vehicle. The majority of other vehicles on the road could slam into a bus pretty fast and cause no injuries to the passengers.