Any long time Moze player can tell you why. Up until recently, Moze’s Iron Bear skill was usually pretty useless so most of the time I played her with guns only and got really good at it.
Hey, sometimes Iron Bear became useful again after some new hotfix or update, but usually soon after some other change would make it useless again. Right now Iron Bear is in fact quite good even in Mayhem 10, but you can’t count on that lasting, and Iron Bear won’t be available in Arms Race.
But so what? If you played her a lot, you know how to thrive without Iron Bear. So bring on the Arms Race!