Because we have something better....parking assist, exclusive in its class in the new Ford Focus. Bold move...putting parking assist your smallest car in the fleet.
I can see why this didn't catch on, it looks like an engineering nightmare. How hard would it be to actually get the spare off to use it if you had a flat?
Also, in the modern era of tiny cars with compact spares and Front-Wheel Drive, this would be impossible.
IMO a cooler gimmick is the BMW that has a remote control so that you can parallel park it while watching from outside the car.