Margins.
GM, Ford, Honda, Toyota, etc. are competing with a commodity product in a commodity market. There's no big difference between a Toyota Yaris, a Honda Fit, a Kia Rio, a Ford Fiesta, and similar cars from every other brand.
They're all within a few hundred bucks of $15k. They all cost do the same thing, cost the same to produce, and I imagine the margins are razor-thin. If any of the brand could lower prices by $500, they'd own the market.
Tesla sells a unique product, with unique technologies.
If Tesla has 5% of the market, but 10x the margins of its competitors, which doesn't seem an unlikely outcome:
1) Its profits will be roughly 1/3 of the total profits of the whole market
2) It will be much more stable. Razor-thin margins mean companies go bankrupt with even minor instability. If Ford's costs rise by 5%, it's dead. If Tesla's costs go up 5%, it's a almost a rounding error.
I think the key question is whether Tesla can execute, but right now, things look promising, although far from certain.