They do a lot of cosmetic damage while not often ending fights except via cut.
I've head butted a lot of guys and knocked out a lot of guys, but I've never knocked a guy out with a headbutt, speaking from strictly personal experience. You're usually smashing the guys nose or mouth with a headbutt, tough to land the "on the button" blow compared to an elbow, but that's all hearsay.That was exactly the line used against elbows for awhile.
I'm gonna suggest it's because headbutt kos are just too gnarly for even us. It's like we all cheer for soccer kicks, then when a promotion brings that back...we're like "ewwww no bro nevermind".