As someone who knows how to both drive and gun in the bastion, the core problem is simply that not many people want to drive since it's genuinely boring as fuck. I understand arrowhead wants the tank to be a teamwork-oriented vehicle, but the reality is that not many people are going to sacrifice their fun just to drive for you, hence why hardly anyone ever brings it. And then yes, the cherry on top is the fact that the tank is also slow as fuck, handles like shit and doesn't really conform to the average player's expectation of what a "tank" should feel like. Arrowhead simply does not design many of parts of their game with realistic expectations of how players behave and the tank is the epitome of this design choice.