There is very low population density between the LA and SF metro areas, which are roughly 350 miles apart (400 miles from city center to city center). This is one of the reasons that a HSR has been such a hard sell.
Big cities with nothing in-between is a good sell for HSR. Europe's two largest countries that have that population distribution pattern (France, Spain) rely heavily on very fast HSR (300-350km/h). The countries with more distributed, but also more dense population (UK, Germany) rely more on fast networks with more stops, but less maximum speed (200-250km/h).