Good question. I would say it’s because when you are trying to sell a car you are selling a fixed thing, and the focus of the conversation tends to be promoting that thing. Don’s style is in selling a dream, a vision of what people want to have or be. He deals in emotions, not facts. Selling a car in a one-and-one situation is about promoting specifications. Selling a car to a mass audience is about selling that emotion.

Also, Don is self-centered, and probably didn’t spend the time talking to the customer to reframe his sales pitch to align to their needs. See question on Quora