i think mostly its manufacturers. they are either suppressing or gaining more money if people still go for the car. they have a limited amount of cars and if they have no issues selling them then they either charge motability customers more or just remove them completely.
at moment at least, motability will have little say.
Current Car: Hyundai Kona Premium EV...2 way 40kg hoist
Last Car: Toyota C-HR Excel Hybrid...4 way 80kg hoist