Population density maps alone will always lead you astray when it comes to building things like HSR if you don't also map in the terrain. It's not super challenging to build light frame buildings and asphalt roads on steep grades, marshes, curvy river valleys, frozen tundra, or even cliff faces. It's much harder to build train tracks, factories, skyscrapers, and other heavy buildings in such places.
A lot of people can live in a place it's where it's difficult to build traditional rail, let alone HSR. Also places where HSR might work can be filled with people and existing construction which you can't always just uproot for rail.
Some countries that built extensive HSR networks (Italy, Spain, Japan) have a lot of challenging terrain.
If you travel, say, from Rome to Florence by Frecciarossa, the line goes through six galleries of total length of some 40 km.
Human settlements are a bigger challenge. People don't like to be uprooted, and small communities along the way don't benefit from HSRs, so they have an incentive to oppose them.
A lot of people can live in a place it's where it's difficult to build traditional rail, let alone HSR. Also places where HSR might work can be filled with people and existing construction which you can't always just uproot for rail.