I think it's not used as often, because it's not always aligned with equestrian goals.
My mare is, has always been, and will always be: bitless. I dislike them immensely. I feel the same about mechanical hackamores. But because of this dynamic, I could never compete in certain disciplines, no matter how much I might ardently desire to.
Also, you need to know your horse, and your horse know you. Some bits can be very subtle. Using a mechanical hackamore correctly can be equally subtle. Not all bitless options are nearly as subtle (some are). And in that way, you and your horse need know each other, so that you may still function with subtlety. Seat, legs, voice, etc.
I show respect to my mare, by using minimal tools. But I show that respect, because our union has earned that type of respect. I cannot say that I would just hop on a random horse bitless, and feel nearly as confident.
It's like knowing you can be bareback, and not have your horse bolt or fuss with you. I happily ride bareback, but again: you won't see me jumping onto and unknown horse without the small security of a saddle.
"We invite you to view wild horses; however, it is unlawful to chase and/or catch them. Please allow them to live a free and unmolested life.
" - BLM website