For the same reason, using Winter would be inadvisable. While his dragonfire was hot and dangerous up close, the dragon was still young, and he couldn't spew dragonfire for too long or from a big distance yet. And even if he could, it wouldn't be very deadly, managing to burn a few men at most before making the enemy scatter. There was always the chance of his dragons dying from a stray arrow through the eye. And if enough arrows pierced a hole in the soft membrane of their wings, the dragon would be unable to fly and would be easily killed on the ground. With time, the membrane would become harder and thicker. But for now, Jon would definitely not let any of his dragons fight in a big battle yet.