You need to stay warm, but you can't have your legs blocked by something like a trench coat (unbuttoned cowboy duster excluded), since you need them free to mount and sit on a horse. A cape keeps you warm, but keeps your front lower body free to ride, and lays flat against the horses back when mounted. Since we ride in cars instead of on horses now, the design isn't very useful anymore. A long cape will get in the way when getting into a car, and when sitting down in a car seat. Jackets and coats are more practical now.