Because it can be very difficult to find affordable plant-based food in cold regions, especially if you're trying to make that food the majority of your diet.

Because it can be very difficult to find affordable plant-based food in cold regions, especially if you're trying to make that food the majority of your diet. People looking to derive most of their nutritional needs from plant-based food need various kinds of infrastructure around them if some or all of those plants can't be naturally grown nearby. Generally this means transporting them from a hospitable growing environment or making the local environment artificially hospitable.