A roll pin by it's shape alone creates radial friction that keeps it in place. A screw unfortunately only creates friction in the threads by interference fit, or by actually stretching the shank to apply pressure. They can loosen, and the user who installs them has to limit the torque or it will overcome the yield strength of the very thin threads.
The M16 doesn't need to come apart for any reason other than a repair, and that repair shouldn't be aggravated by stripping the threads. Stoner and the crew knew what they were doing when they picked the specific fasteners and why. Roll pins pretty much go in once at the beginning of the weapon's service life and aren't removed until it's arsenal rebuilt to extend it's life.
It's the civilian shooter who changes out parts and furniture frequently who prefers simpler fasteners they understand.
If you want a good look at how well bolts and screws work, you could visit a busy traffic intersection and see them lying on all sides of the road. Bring a magnet, you'll pick up a pound of them and that will likely be one screw or bolt less that causes a flat.
I'll stick with roll pins.