They don't really. It is just that these specific cultures offer thesse specific MW weapons and are very distinctly identified with those cultures. Like the Vermillion Bowmen bows look like an "X", and are very ornately carved with symbols of the bearers accomplishments and/or famous ancestors.
Just cool cultural histories and distinct markings to go along with these weapons.
This setting really pulls on a lot various cultures, then twists them to be not like you would expect. Like the Mountain King is an evil dwarf who uses Orcs in his armies and rules over a half million dwarves, orcs, humans, etc...
Plus good aligned reptilian people, and the Central/South American flavored cultures, and lots of other stuff.