Outdated laws from the dawn of the car industry. Ford and GM were a lot smaller and definitely couldn’t afford to have a show room in every state and market that might be interested in cars. An industry of small businesses called car dealerships was born.
One worry was that an original mom and pop dealership in a particular area could build a brand, and create great business and awareness for a particular brand, only to have that brand open a direct sell show room after all the risks have been eliminated. They campaigned states/local governments to pass laws prohibiting direct selling or more commonly (I think) direct selling within some large radius of an existing dealership for the same manufacturer