Why does geographic variation in sales persist?

I've put a blog entry on the Brand Science Institute site about some work in this area.

http://www.b-s-i.org/blog/?p=138

The work begins with Bart Bronnenberg (and his colleagues Dhar and Dubé), who note that there's often substantial variation in who the leading brands are in a category. For example, Folgers might be the larges coffee in one market, while Maxwell House is the largest in another. These differences are large and persistent.

I focused on WHY these differences are persistent. For more, see the BSI link.