There is only 600 km between the two cities (I'm french distance between SF and LA was unknown to me before :) ), a normal train (200 km/h, with TGV it would be 2h and some) would take a bit much than 3 hours; with google map I only see fields, desert and mountains for nearly 550 km, why sleep in a bus ?
Because Republicans hate public transit. (And, to be fair, we rather suck at infrastructure for a variety of complex reasons, which explains part of the Republican resistance to such projects.)
HSR ends up being a heavily subsidized option for upper middle class people and business travelers. (Look at who is in a typical Acela or even Northeast Regional train.)
So to be more precise, there is no HSR in California because (1) the fact that we suck at infrastructure projects makes it a very expensive proposition; (2) that taxpayer funded expense would go to benefitting a relatively small and well off set of people living in SF and LA.
(Also, which “republicans?” California doesn’t have a single republican in elected statewide office.)