It used to be for stability – dropping one knee provided better fore and aft stability than the parallel position, especially when wearing soft, leather lace-up boots. With good telemark technique, the backcountry became a ski playground long before AT gear became as reliable and lightweight as it is today. Only recently, with the invention of ultralight Rando-racing technology, does fixed-heel ski gear compete with the weight advantage of old school tele gear.
SCARPA climbing shoes
SCARPA Ski Boots