Higher population densities make bullet trains more attractive there.
The national average doesn't count. What counts is densities over size scales of a few hundred mi/km. Consider China. Most of its high-speed lines are being built near its east coast, where the people are. There is exactly one in Xinjiang and zero in Tibet, neither o which are very populous.
HSR in the US will develop in the same way -- the lines being built where the people are. California is thus a good place, as are the East Coast and the area around Chicago. By comparison, a high-speed line through the Rocky Mountains would be folly.