I mean, the American people didn’t abandon them. GM used monopolistic practices and corporate collusion to basically make most major cities an offer they couldn’t refuse. The gotcha, of course, is that they were being offered a “sweet deal” on a transit mode that is overall less effective for major passenger corridors in large cities, and have shorter average service lives, and use consumable parts much more heavily:

The General Motors streetcar conspiracy refers to the convictions of General Motors (GM) and related companies that were involved in the monopolizing of the sale of buses and supplies to National City Lines(NCL) and subsidiaries, as well as to the allegations that the defendants conspired to own or control transit systems, in violation of Section 1 of the Sherman Antitrust Act. This suit created lingering suspicions that the defendants had in fact plotted to dismantle streetcar systems in many cities in the United States as an attempt to monopolize surface transportation.

Between 1938 and 1950, National City Lines and its subsidiaries, American City Lines and Pacific City Lines—with investment from GM, Firestone Tire, Standard Oil of California (through a subsidiary), Federal Engineering, Phillips Petroleum, and Mack Trucks—gained control of additional transit systems in about 25 cities. Systems included St. Louis, Baltimore, Los Angeles, and Oakland. NCL often converted streetcars to bus operations in that period, although electric traction was preserved or expanded in some locations. Other systems, such as San Diego's, were converted by outgrowths of the City Lines. Most of the companies involved were convicted in 1949 of conspiracy to monopolize interstate commerce in the sale of buses, fuel, and supplies to NCL subsidiaries, but were acquitted of conspiring to monopolize the transit industry.

I’d love to do the Am track one day looks like a great way to see it all.

I have wanted to AmTrak the country for like 18 years now. The thing is it's just not an appealing experience for the cost. A trip from Florida to California would take 120 hrs/4.25 days one-way. That means that someone would need to take 9 days for a round trip if things match up perfectly. The other thing is that tickets can be quite expensive. The same trip is ~$550 one way. So, we'd have someone spending 7 vacation days and $1100 on transportation alone to sit on a train in coach for nearly the entire time without even getting to see their destination. Say you wanted to stay a week in California. There go another 5 vacation days for a total of 12 vacation days spent, with about half of them spent on a train in coach. You'd also have to add in the costs of staying and touring California, which can be fairly cheap if you know someone there or very expensive if you don't.

Very few people in the US have the time off and the financial means to make this an appealing trip.