Maybe because it use less metal ? That was the point of axes at the beginning, they use less metal than swords.
well it's not just less metal (although that's the main reason) but also because you can get away with using worse quality/less pure metal, and it's much easier to make a sharp triangle that doesn't need any flexibility, heat treating, etc. compared to a sword which is a 3mm thick stick that will fall apart if the metal is bad or it's not made correctly
anyways if I had to take a guess these were probably ceremonial, as it would be a liability in combat as well as probably mess up cuts