For the reason Hugomath stated. Besides, if we would trigger all tanks to not really die, but just remain as a wreck, the map would quickly be cluttered by dead tanks, and it would be a pain to have them, for instance, blocking a bridge or other chokepoint and most of all: we don't want for instance allied players to have axis vehicles since that confuses the players (since we don't have teamcolor)
Again i think you are missing the scope of this map just a little, this would be really sweet to have in a Mow (where i assume you saw it before) or CoH type map, but not in this.
But anyways, @Vuorma and Hugomath, i DO think that winter effects are a great idea, actually i discussed it with a friend today (who is really interrested in the map btw) and it is ofcourse VERY possible to add/remove the effect or adjust it's strength depending on season. Tha tihing that got me confused though is the whole issue of Russian winter vs Finnish winter, it would be strange if the finns were unused to the Russian winter, while the russians were unused to the Finnish winter! Having one nation being immune to winter effects is really simple, you just make the buff dummy unit belong to the player that is immune and make the targets allowed of the aura to be "enemy, ally", but not "friend". However, with more units it gets harder, and perhaps the best suloution is to make al players equally affected. :S