Why is G.M. dropping Pontiac?

A look at the 2008 sales for G.M. shows that Pontiac out-sold Buick by more than two-to-one (world-wide). I doubt that Buick has out-sold Pontiac even once in the last 60 years. How can G.M. justify dropping Pontiac and keeping Buick which sold an incredibly meager 27,643 vehicles last year?