It's a combination of access to cheap military surplus ammo, light recoil making them approachable to beginners, a standardized platform, and rails for accessor

It's a combination of access to cheap military surplus ammo, light recoil making them approachable to beginners, a standardized platform, and rails for accessories. Plus a little bit of a "badass" effect from it being based on a military weapon. Military weapons also generically have the advantage of having been tested for ruggedness, and not having to worry about being able to find parts or ammo in a decade.