I think that diagnostic work on all makes is too complicated to successfully franchise.
The expert technician with many years of multi-make experience can probably make success of it, although there are probably easier ways to earn a living.
Many franchisees seem to take on a franchise with little previous experience, often with their redundancy pay.
I suppose a franchise is a good thing is for someone who is technically proficient, but with little in the way of financial or commercial experience.