Why is the protection fighting style considered subpar in D&D 5E?
Nobody has determined that, so it cannot be concluded that the protection fighting style is in any way bad.
It is true that in this edition of D&D the “defend” action use a move for its defensive capabilities which means you can’t take an opportunity attack when defending. However, since players have to declare who they are defending against at the start of their turn, it’s relatively easy just not to defend and instead exploit your opponent’s position with an opportunity attack or using some other damaging ability–even if you don’t do much or anything at all this round. And afterwards on your next turn you can then choose to use a protection fighting style to protect yourself, without any lost opportunity attacks.
