It really is that car dealerships are very wealthy and have a lot of political clout. There were also early rulings that a manufacturer can't order a car dealership how to operate - the basics of the case were that American manufacturers were lending or outright granting money for people to start dealerships. Those dealerships started trying to sell foreign cars, the manufacturers said "no way", the dealerships fought back, and the dealerships won.

Basically it's become a textbook example of the entrenched parties setting up barriers to entry. They have lots of money, and are often active in their local communities, so people generally like them.

Edit: basically, I'm saying it really is too hard to change because the law is on the dealerships side, and there's a lot of momentum and status quo to overcome.