Population density. Places that have great trains (many European countries, Japan, and the U.S. Northeast) have high population densities.
Average population density of an entire country is not a factor when the primary goal of fast trains is connecting major cities of great population density while traveling through agricultural areas of low population density.
Sweden has its X-2000 tilt train (one of the trains tested prior to the “Acela”), and its average population density (54/mi²) is lower than that of the USA (84/mi²).