Middle men don't like confronting their own mortality. If Tesla succeeds, they can't sink their fangs into the revenue stream. If Tesla really succeeds over a longer period of time, the other car makers might start getting ideas.
Second, dealerships make their money off of the maintenance as well as the sale. Allowing the manufacturer to dip into that maintenance revenue stream will create perverse incentives that hurt consumers.
The first one doesn't really apply, as Tesla has the capital to start all of the dealerships they need and that's basically what they want; they don't want 3rd party dealerships popping up at all. The second one applies to Tesla, but I don't think it really matters. They are The Experts in electric cars right now, so any significant maintenance will probably just get shipped back to their factory anyway.