It's not only the time and distance of the journey itself that matters, but also the layout of the cities and the infrastructure on either end. Acela works (despite the infrastructure it runs on being crap) because a) you can actually get places without a car from Penn Station or Union Station (the NY to Boston stretch has no competitive advantage over flying, so it's excluded), and b) most important destinations are a short walk/cab ride away. Plus, there are big intermediate cities with strong demand like Baltimore and Philly to keep trains full. HSR does not work if too many people have to rent a car at their destination, because they could just fly to the airport (which is still faster and cheaper) and rent a car there instead, and get right on the Interstate rather than driving through busy downtown streets they're not familiar with. What's the point of having HSR to downtown Dallas when travelers' destinations are going to be all over the Metroplex, and you can't get around without a car? Florida and Las Vegas can only work because of the high number of tourists and conventioneers, who basically go to the same places.
The thing is, most business travelers and tourists are going to CBDs, even in America. Going to Atlanta for work? You'll probably be in Downtown, Buckhead, or Midtown. Going to Pittsburgh for school? You'll probably be in Oakland. Going to Los Angeles for fun? You'll probably be in Downtown, Hollywood, or Beverly Hills. And even if you are headed to the 'burbs, most HSR stations are closer than airports which make them pretty compelling if you'd otherwise fly. Going to see your mom in Norwalk? You're better off ubering or renting a car from Anaheim or LA Union Station than LAX.
And Acela is "fast enough", which is all it needs to be. The average speed of the nonstop Acela is 87mph between New York and DC, which is faster than the Paris-Toulouse TGV, and all but one (once-daily on weekdays, one-way) Edinburgh-London LNER train. The problem is that the Acela's price premium cannot be justified over the Northeast Regional or similar trains for all but business travelers, especially if you're getting off at an intermediate station.
It really isn't though. Self-respecting high speed rail averages well above 150mph. Sure, Amtrak might have a near-monopoly on Manhattan-DC traffic, but if it took 90 minutes from NY Penn (or Newark or Metropark or...) to DC (or New Carrollton or Alexandria or...), suddenly it gets a majority of the fly/drive/rail market from all of NY-NJ-CT to all of DC-MD-VA. The Northeast corridor is a bigger market than Tokyo-Nagoya-Osaka. It can support the best of the best.
One final point: In the Northeast, you're underrating the enormous pull of New York and how deeply undesirable it is to go with a car. Most people going to NY are going to the Island of Manhattan, and it's an awful lot of people. If you get serious HSR in on DC-NY, suddenly trips like Raleigh-New York, Pittsburgh-New York, Cleveland-New York, Detroit-New York, Toronto-New York, Charlotte-New York, Columbus-New York, Cincinnati-New York, etc. become viable in under 3 hours as each little addition to the network opens up travel pairs across the network. The same goes for Chicago, where most people are headed to the Loop and DC, where most people are headed to the L'Enfant City. These cities are bigger pulls than London, Paris, Madrid, Milan, etc. The amount of inbound traffic to these three cities alone that doesn't want a car on the far end is enormous, and the closeness of each city is such that just 100 miles of new rail can add 4 or 5 top-tier city pairs. But if it's gonna take 3 hours from DC-NY, you can forget about OH-NY or NC-NY or IN-NY.
