The regulatory capture argument someone made below is hard to square with the history:
http://www.americanbar.org/publications/franchise_lawyer/201.... At first, cars were sold directly to consumers by manufacturers, and independent dealerships were rare. It's really hard to explain using a regulatory capture or lobbying model how car manufacturers, huge incredibly profitable companies at the time,
lost their bid to preserve the status quo.
In reality, these were consumer and economic protection laws. States didn't like how much money was flowing from their economies to Detroit, and by making direct consumer sales illegal they gave a lot of leverage to franchises, which are local businesses. They also justified the laws on consumer protection grounds--local car dealers would be a lot more responsive to customers than a megacorp in Detroit.
As for the "planned economy" aspect--remember that the ideas of what constituted unreasonable interference in the economy were very different back then. The revolution of applying economics principles to government didn't really happen until the 1960's and 1970's. Before that, quite extensive interventions in the economy were routine. For example, prior to the 1970's, the Civil Aeronautics Board set airline ticket prices! That would be amazingly controversial in the U.S. today, on either side of the aisle, but was par for course back in the day.