The consumer-based reason is that the imports have a higher perceived value.
Another reason that applied until GM and Ford changed their strategy was that in order to keep volumes high they sold a lot of units to fleets, for example rental car companies. The fleet customers keep the cars for a couple of years, then sell them. This creates an oversupply of two-year-old used domestic cars, which depresses price. The loss of resale value erodes their retail customer base, which makes them even more dependent upon fleet sales. It's a vicious cycle, one from which GM and Ford tried to extricate themselves.
Chrysler embraced fleet sales wholeheartedly as GM and Ford pulled back from them. This makes sense for Chrysler since no retail customer in his right mind would buy one of their products, while fleet customers care mostly about price as opposed to quality.
(This doesn't take into account the loss of business some rental companies suffer as renters steer clear of them in order not to be stuck in a POS Chrysler.)