Top U.S. "Clunkers"

In the recent federal "cash for clunkers" Stimulus program, multiple thousands of low-MPG vehicles were traded in, ostensibly both improving our air quality potential as well as stuffing some much-needed cash into America's automotive industry.
However, seven out of the top 10 "clunkers" traded in where FORDs, the rest being Chrysler products and no GM clunkers.
If GM products were so bad, why were consumers apparently hanging on to their GM potential low-MPG "clunkers"? Or, given that the vehicles traded in were all basically 10 years old or older, does that mean equivalent GM vehicles had rusted out and are no longer even road-worthy enough to be traded in?
This is especially interesting, given that Ford is the only major U.S. auto maker that didn't declare bankruptcy.
If I were in Ford's PR department (or more to the point, Ford's engineering design department), I'd be wondering about this one.