I think part of it may be because all traditional "martial arts" are, for the most part, obsolete. "Martial art" means "art", in the sense of craft, and "martial", in the sense of going to war. If you're going to war, being really good at punching and grappling is useless compared to being passable at those things but also having a rifle and knowing how to use it, especially in coordination with other riflemen or combined arms.