You are never going to get people to walk from their million dollar gated communites to Nieman Marcus. It defeats the point of thier 'Benz. Most of the neighborhoods within a 15 mintue walk of the mall are very ritzy and these people wouldn't be caught dead walking to get their mail, let alone the mall!
I work across the street (carnegie blvd) from the mall I the only people is regularly see walking the sidewalks are the folks from the projects. Really there are not that many residences THAT close to the mall to encourage walking. It is improving with Piedmont town center, the residence, and the Earth Fare project (and the many proposed developments) being built as these residences are not the millionaire homes.
Another thing people talk about is SouthPark traffic. Honestly, it isn't bad at all. I drive through it 4 times a day and it isn't bad. Until traffic (and parking) becomes a nightmare like around Phipps and Lenox, people will continue to drive 'cause it is easy!