Part of it is that the manga which achieves the most popularity tends to be shounen fighting stuff. I mean, look at the top four: Naruto, Bleach, One Piece, and Fairy Tail. They all use much the same formula, but with the details changed. (Naruto has ninjas, One Piece has pirates, Bleach has something else, and Fairy Tail has mages.) It gets tiresome after a while, but that kind of thing tops the charts, consistently.
Meanwhile you have absolutely beautiful manga like Yokohama Kaidashi Kikou (which is made from liquid happiness) that don't even get licensed for official English translation.