1. Few people People tend to not go out in the winter. So the zombies will have to walk door to door to infect people. This would be a long be boring process. Very few would be infected in the first place. 2. Harsh weather Zombies are clearly not the strongest. So when the cold frostbite their head off, they will be useless. Even fewer zombies! The heavy weather also slows them down. Bonus! 3. Snow If you heat up snow, you get water. During the winter you get free snow. You can have enough water to last quite a while, which is important, duh!