It costs money to train people to shoot at stuff, and not everybody drives one specific type of car.
It's easier to hand them a gun that can shoot through stuff easy and tell them to "aim at the engine" than to train them to stop a car with a smaller gun. Rifles are bulky and don't work well in short range combat. It's easier to fight with a pistol overall because it's inexpensive and easy to use in combat.
A .50 round is about 3 dollars.
A 5.56mm round is only about 25 cents.
>inexpensive
A desert eagle can tear your tendons if you don't know how to use it properly.
An average AR15-style weapon is reliable at not breaking your arm and ripping/tearing your tendons/muscle.
>easy to use