they only have one flap because they dont use the other flap...when theyre batting, if thayre a righty then theyll have on the left because they wont get hit on the right side of their head
This is for a couple of reason: 1. They don't have to share with anyone else. 2. One ear flap helmets are much more comfortable to wear and you can easily take it off with one hand without ripping your ears off.
Even switch hitters use one ear flap...they just have two helmets.