Finland has a short growing season and with everyone planting at the same time the crops mature at the same time meaning you'd have to buy your entire years supply of whatever all at once, and store them, or the farmers have to store the produce and dribble them out over the year.  My guess is most people don't have the space to stock a couple of tons of veggies and the farmers cannot afford to purchase the storage equipment and absorb the costs of running 'em.