Well, Porsche just has that racing/sports car mystique, and Audis don't.
I know that there must be some people that don't know what an Audi is, which probably contributes to the effect. EVERYONE has heard of Porsche.
I met one that apparently hadn't heard of Audi before at a stoplight driving my A4 a couple months ago. Some middle-aged lady in an Explorer in the left turn lane on my left at a stoplight, yelled out:
Lady: "What car is that?"
Me: "An Audi A4."
Lady: "A what?"
Me: "An Audi."
Lady: "Who makes it?"
Me: <pausing, not sure what to say> "Audi."
The light, by this time, mercifully turned green and I took off. She was curious about the car, because she backed up in her lane then pulled forward like she was checking out the badging. A4s aren't rare and have only been kicking around for the last 13 years. Some people...