You know, the other thing is, snow is hard to walk through. Especially deep snow. Now we didn't really get much of that here, per se, but further north, or in those winters when we have a couple of feet on the ground, it's hard going.
And when I say "deep" it doesn't have to be to your waist to make it hard going. Two feet, three feet, and it really slows you down because you sink into it. Granted, zombies can just keep going, and going, and going, like a bloody Energizer bunny, but it would also prolong their exposure to the elements, assuming they weren't smart enough to get out of the weather.
Which, in 90% of the zombie stuff I've ever seen, they aren't.
ETA: Thus making Canada, Scandinavia, and Russia the last outpost in the zombie outbreak, at least in the northern hemisphere.