The reason is becuase names pick up bad reputations. It takes years to gain a good reputation in the automotive industry, such as "Camry" and "Accord". It takes only a couple years of bad publicity, reliabilty, and recalls, to make a car get a bad rep, (and thus dropping sales) A good example of this is the Citation. The first model year, they sold over 840,000 untits! That's almost a million in ONE year! Every year thereafter, they were almost unable to sqeeze out even 100,000.
And even though "Cobalt" has only been on the market three years, it already has gained itself a bad "vibe" from buyers. Just goes to show how subsatndard American car companies produce their products these days...