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.