I may not have the full answer but I think I have part of the answer. The Big 3 have it in their contracts with the UAW that they will keep so many dealerships open. Each dealership has to have a minimum inventory thus providing a minimum number of cars being produced each year (job security).
Bankruptcy and government takeovers have freed Chrysler and GM from some of the limitations of those contracts. They dont owe anything to the independent, privately owned, non union dealerships.
That’s the only explanation that makes any sense at all.