I think population density is a factor. China and the US have the same area, but China has 4x the population. Consider also England and Japan, densely populated islands with great train service.
Japan > England >> USA. Just being able to take the TGV from London to Paris is mindblowing.
I define great as it being generally possible to get where I want to go on the train (this is the most important factor) and then as a bonus it's usually competitive with driving, busing, flying, cycling, or walking in terms of comfort, cost, and convenience.
China is heavily trying to also connect its less populated areas to proper train services.
They just built a new highspeed train to the Xinjiang region which is a very sparsely populated area in the north east of China.