Here’s a better theory: because American public transit is, when compared with the alternatives, not safe, not clean, and not convenient. Take LA, probably the most car-dependent big city in America. Riding the bus or subway in LA is not an enjoyable experience. Nor is it enjoyable to walk around the areas where the stops are. If I were trying to get more people to use public transit, I’d start by making the stations and buses/subways beautiful, clean, safe places that are just nice urban places to hang out in. There’s no need to make it a moral crusade; just offer a better product and more people will use it.
The thing is, the entire society (at least in Copenhagen) is built around car-lite life (for example small corner grocery everywhere instead of large supermarkets). Additionally there is such low abject poverty that there is little tension with crime, homelessness etc.
My point is, lack of interest in public transit is merely symptomatic of larger issues we as Americans face, such as sprawl, existing infrastructure, crime, inequality etc.