The NEC is a legacy 100 year-old infrastructure, whereas the Calfornia high-speed rail project is clean-sheet design. There was never a rational explanation as to why California should use NEC-compatible equipment when the corridors are so completely different. And now, thankfully, sanity has prevailed:
It became clear in meetings with manufacturers during the last few weeks that the requirements were too different to incorporate into one set of trains, said Lisa-Marie Alley, a spokeswoman for high-speed rail.
“The feedback that we got from the industry was that Amtrak and high-speed rail need such different things, it was almost impossible for them to build a train that meets both our needs,” she said. “We’d hoped that the industry had evolved to where they can accommodate both.”
The agencies concluded that too many compromises would need to be made to meet both their needs, which would “move us away from a service-proven design and create significant risks as to schedule and costs,” Amtrak spokesman Craig Schulz said in an email.
One of the puzzling things about the CHSRA has been its inability to work with its California partners (Caltrain, Metrolink) on really basic things, like compatible platform heights and signal systems — while at the same time design its high-speed trains to be compatible with a rail line 3000 miles away. The NEC requires high-platform trains, which precludes Caltrain and CHSRA from sharing platforms. Hopefully, with this decision, Caltrain and CHSRA can at least use trains with the same platform height.
Systemic Failure 
I believe high platform trains are required for all high(er) speed trains, not just NEC trains which has caused some friction in various places where Amtrak has started running 110mph+ services between cities previously served by slower trains in States that standardized on low platforms. So CAHSR will need the high platforms anyway.
(Not that this is a bad thing, and while it can’t be done overnight it probably makes sense for commuter operations to move towards high platforms anyway for safety, accessability, and faster boarding reasons.)
High platform stations either must be on tangent track of have a wide gap between car and platform. Artistic renderings of stations on curved track say in effect that either the gap is large or the platform is low.
High platform stations either must be on tangent track or have a wide gap between car and platform. Artistic renderings of stations on curved track say in effect that either the gap is large or the platform is low.
Sorry for the type above.