From reading a lot of motoring journals , I believe the main reason Tesla are not on the list are the costs and timescales surrounding repairs and servicing.
Tesla have very few repair centres and are fiercely territorial about 3rd parties touching their vehicles. If a Tesla is damaged it will be off the road for longer than average, parts will have to come from factory so overall, much more expensive and slower to repair than other cars. That is why many private owners have found their insurance premiums rocket and some Insurance Companies refusing to insure them.
Why would Motability get involved in this headache when there are many quality alternatives?