The primary reason (as far as I'm aware) is because buses are (relatively) more massive and structurally different from cars, so they exhibit different collision physics in the event of a crash. In the event of a crash, the greater mass means any forces can distribute across a greater load and lead to less acceleration for those inside.

I think there are more details related to types of crashes (e.g. head-on vs on the side) that are taken into account through a bus's structure and how it's internally designed too, but I'm less familiar with how and why it works.

My understanding is that buses generally reach lower speeds and, because of their mass, a collision is less likely to produce the kind of abrupt stop that throws people through windshields. If a typical car collides head-on with a bus at speed, the car will be thrown back and its occupants whipped around; the bus will keep moving forward with an unpleasant but not devastating jolt.

There are exceptions, I'm sure.

I think the only explanation I've heard for this is that seatbelts are too expensive to put on every seat in a bus. Interestingly, on mine sites the buses all have seatbelts and you're firmly instructed to put them on every time the bus starts moving.

High mass, they don't slow down quickly. There are a few papers out there. There are very few things that can decelerate the mass of a buss quickly