For one thing, the car itself is stylish and attractive, which means consumers don't have to look like they're driving around in half a car in order to cut pollution. Add to that the celebrity factor-luminaries like Leo and Gwyneth proudly tooling around town in their Priuses-and you take care of any lingering image concerns non-environmentalists might have.
Keeping image first meant bringing in the environment second, which was a masterstroke. Because everyone is an environmentalist at heart, the eco-friendly aspects of the Prius became more of a tie breaker for consumers than a motivating factor. All things being equal, who won't choose the car that wreaks less havoc on the environment and saves you dollars at the pump?