It's winter, and up north the air is freezing. Zombies are less of a threat right now than the sickness, cold, and starvation that wracks us. When the snow lets up, that is our chance to go and scavenge. However, it seems spring is approaching, as it snows less and less.
The dead are almost as frozen as the snow. They are sluggish and seemingly less active. However, there are many of them, because the colder it is...well, the less survivors are actually surviving.