Tank, as a projectile weapon, doesn't really add much to the game. Either we would have to change it to make it practical, and it wouldn't be the cheaty fun that it was, or it would just be a no-fun-zone. Rocket Arena already gives us what Tank as a weapon could, without ruining balance.
Tank, as a vehicle, requires an appropriate place to drive it around. This eliminates most maps, and the few maps that can support it would see it more as a gimmick than a useful addition.
If/when single player gets rolling, tank is probably going to happen, but don't expect what is truly a parsley garnish to suddenly become the main course.
