In regions with harsh winter and insufficient logistics to transport things around during a snowstorm(ie. pretty often even today), going vegan just isn't possible.
You can't keep your veggies fresh that long even with refridgeration, fermented vegetables(while good) can't compose your ENTIRE diet, lack of fat will make you significantly more susceptible to the cold, bread/grains alone is extremely inadvisable for your health(even if you managed to choke all that down without butter or milk), berries are expensive and difficult to stock up on(they used to be more expensive than meat), and there simply isn't enough total mass being grown without large-scale agriculture(unless you're fine with eating grass).