Perhaps some of the 'guns' Lucca uses and the 'arms' utilized by Robo are just COMPONENTS that 'transform' their weapon into "Stone Shot" or "Terra Arm", etc. You know, they are personalizing their weapons (such as Lucca making her gun capable of firing dreamstone shot). Just as we rarely see Crono and friends eat, sleep, or go potty during their adventure, we may also not see the heroes re-utilize their weapons with "upgrades".
Any who, the Dragon Tank probably got shut down after two teenagers destroyed it. If a military weapon was easily destroyed/dispatched by two teenagers here in the good ol' states, I think the military might view such weaponry as sub-par. Most likely was the case with the Dragon Tank...
But the technology put into the tank is obviously important. As mentioned before, lazers, flame throwers, etc. Plus, when you defeat the Dragon Tank's head, we clearly see advanced machinery beneath the plating. Also, it doesn't seem like the tank was piloted, so most likely it was unmanned and had sophisticated AI enabling it to target enemies and make proper battle tactics.