I think it's because WTF TKD i.e. olympic style taekwondo is meant for sport. I saw school in korea training and they had a session, which they were punhcing, elbowing, and backfisting at each other. Funny thing is I hear people say this martia art and that martial art, but since I've seen taekwondo fighters in asia be successful against MT fighters I've come to the conclusion that it ultimately comes down to how you train and what techniques you emphasize.