Because no one will ride that section. If you start on the ends people will actually pay money to ride the train, which then funds construction on the other part of the train and provides steady jobs instead of only temporary ones.
If you want to provide welfare to people without jobs, then provide welfare. Don't provide secret welfare and call it a high speed train.
> If you start on the ends people will actually pay money to ride the train
The initial operating segment runs through the Valley to San Jose; that's pretty close to the S.F. end , and is the third biggest city in California (and the second biggest covered by the whole Phase 1 LA-to-SF alignment.)
Obviously, the initial operating segment isn't going to connect both ends, and short of doing that, where specifically would be better than the actual planned IOS?
Sure, the initial construction segment is a small section in the valley that would probably see little ridership without being connected to some place nearer to the endpoints, but the ICS is not the whole IOS.
Politically it would've helped the project much more to have had tangible success in the economic heart of CA, impacting shitloads of people (and voters).
If you want to provide welfare to people without jobs, then provide welfare. Don't provide secret welfare and call it a high speed train.