A helmet doesnít protect well from commonly used combat rifle rounds.

Helmets are heavy.

They cannot be easily packed away when not needed.

They donít protect from any shot in the face or the back of the neck.

About the only thing they are good for is protection from shrapnel, falling debris and some hits on the head from above by a blunt object. If the soldier is targeted by a sniper wielding a .50 BMG or .338 Lapua Magnum rifle no helmet will make a difference.

I once interviewed a French Foreign Legion war dog who was fighting in Bosnia. He told me he never wears a helmet or flak jacket (bulletproof vest) because it slows him down and hinders his movement. He said that the best protection from being shot is to be constantly on the move.

Source: https://www.quora.com/Why-do-modern-...ead-of-helmets