The problem could be that not many people have actually written/e-mailed/telephoned Motability to formally express their displeasure with the choice of vehicles available on the scheme, (I don’t really recommend telephoning as whilst the call handler may be sympathetic, the message rarely gets beyond the individual call handler).
It is one thing complaining on an internet forum about choice, but Motability could well be oblivious to it.
If an awful lot customers started writing to the new Motability Operations CEO (Andrew Miller), formally expressing their displeasure, something may happen (no guarantees though). But at least the management of Motability Operations will get the message that there are a lot of very unhappy customers.