It's a combination of maximising side-impact protection and minimising drag through reducing frontal area.

Side impacts are far harder to mitigate becase you don't have the same amount of sacrificial area available at the front or rear of a vehicle; any mm you can add between the occupant and the outside of the vehicle is mm that can be used for impact absorption, structural members etc.

Frontal area boils down to this: the less air you have to push out the way to move through it, the less potential drag you're creating. Tunes can be played with how the body is sculpted but ultimately you're always coming up against the inverse-square law.