Entirely cultural reasons.

1. ⁠We are overall extremely hesitant to invest infrastructure. We prefer to let things come close to outright collapse, or just let them actually collapse, before spending any money on them.
2. ⁠We are obsessed with personal automobiles and prefer to do most of our movement around where we live with them. This makes any other system of transportation very difficult to implement. Ridership will initially be very low, which creates cost challenges. And the population is very hesitant to accept any compromise in freedom of driving or subsidies for driving that might benefit other means of transportation.
3. ⁠We are also, relatively speaking, obsessed with individual land rights and hesitant both to give up land cheaply or to use existing eminent domain powers to buy up land and use it for infrastructure.
4. ⁠Historically, we fear cheap means of transit that connect areas of different socioeconomic profiles, due to the fact that it provides access to wealthy majority areas for racial minorities.

I think we’re capable of overcoming all of these, but these are the real bedrock issues Americans have with mass transit. We don’t like to plan ahead, we don’t want it to get in the way of us driving everywhere, we don’t want infrastructure on or near our land, and we don’t want racial minorities to be able to go to the places where we live.