Four reasons I think
(1) It takes a lot of work to gain initial approval of a type, but once one of a type is approved, everybody else can jump on the bandwagon.
(2) It takes a lot of money
(3) There are very few Engineers in this country with the ability and authorisation to "recommend" to CAA that an aeroplane, new to the civil register, should be allowed to fly.
(4) Spares supplies vary enormously depending upon type, age and how it was retired from service by the military.
G