Headbutting is a very effective striking technique. Done right it is fast because it doesn't telegraph much and there is a lot of bang for your buck. I notice in MMA contests that fighters adjust to head 'rubbing' and usually have short brush cuts. It is like the difference between a lock and a break - velocity.
I think the crowd/entertainment factor is also a consideration. Hit a guy with a few head butts, fight is over in 10 secs. If every fighter started using head buts it would make a short night for the ticket price.
Safety would definitely be a big consideration as well.
Paul M