Because geared hub drives can short the winding when forced to climb too far too slowly. Mac said 1000' in an hour. Major brands hope to sell in 50 states. Only CA, ID, CO, MT, OR, WA have such mountains, but serious quantities of ebikes have been sold there. Warrenty claims do not make for great PR.
DD hub drives use too many watthours climbing mountains, accelerate slowly, and generally do not give a positive customer experience in hilly areas or stop & go traffic.