The difference is their helmets are hard externally providing minimal impact absorption and have a whiplashing action when they bounce off each other. Our headgear is made from spongy material which absorbs the impact.Headgear does nothing for concussion. Look at the NFL, helmuts don't do much
Simple experiment - drop a marble or ball bearing from a height onto a concrete floor and see how high it bounces up. Now put a 1cm kitchen sponge on floor and drop the ball from same height onto sponge. The sponge has absorbed some of the energy and the ball does not bounce as high.