The same reason horse people do plenty of other things.... "thats the way its always been done" who cares if its the best thing for the horse... ok sorry that sounded pissy
. Really though, I think everybody else about has it. A combo of tradition and the easy way out. Out of my three walkers the one who walked the best was ridden in an eggbutt snaffle. The others had "tennessee walker bits" I wish I knew now what I did then about bits and it wouldv'e been a different story.