There's definitely parts of the US where the population density could easily sustain HSR, though; California from San Diego–Los Angeles–San Francisco is one, and Boston–New York–Philadelphia–Baltimore–Washington is another (quite how far you could stretch that network west is an interesting question, and I don't know enough about current travel trends to comment, certainly Pittsburgh is close enough that travel time-wise it could beat out aviation), as is Miami–West Palm Beach–Orlando for example.

What is interesting is it is likely the case that the North East is the only place where there's a sufficient population density in a larger general area that crosses state lines.

The problems seem to be frequently the fact that nobody wants a train line near them that doesn't stop near them, whereas current aviation routes simply overfly them, and adding in extra stops completely destroys the business case by making them comparatively slow (v. aviation).