This week Tesla has been in a battle with New Jersey Governor Chris Christie over a proposed law that would ban the electric car manufacturer (or any other car company) from selling directly to customers. This isn't the only state where Tesla has battled/is battling similar legislation. So why doesn't Tesla want to sell through dealerships in the first place? Elon Musk explained the reasoning in a letter addressed to people of New Jersey.
Musk says there's a conflict of interest when dealerships that primarily sell gasoline cars also offer electric ones. Given that the former takes up the vast majority of the market share, the dealerships have an incentive to push the gas-powered cars. It's just easier. He also points out that no other startup car manufacturer—electric or not—has been successful in selling its cars through dealerships. Just look at DeLorean and Fisker.
Further, Musk points out a larger conflict of interest generated by dealerships profiting mostly from service. Teslas don't require nearly as many repairs as gasoline cars, and even if they did Elon doesn't want to generate profits when his product "breaks."
The letter also delves into the background and reasoning of the proposed law, as well as what Tesla plans to do going forward. It's worth a full read if you have a bit of time on your hands.