One of the reasons Brightline West was willing to take on the Las Vegas scheme was that Desert West had done all the heavy environmental lifting. So BLW had a pretty good picture (at least between Victorville and LV) what they were up against, and the changes they decided to make reduced many of the impacts identified. They then did some rethinking and figured out that the compromises inherent in sticking to the freeway were acceptable for something intended to make money without waiting forever, and were able by doing that to figure out how to extend into the Basin for a reasonably complete line (which Desert Express essentially left for somebody else to figure out). For that matter, if they want to go to Palmdale, the fundamentals have already been done by Caltrans and Metro, too.

A similar situation does not exist for LOSSAN. Nobody has done a comprehensive long-term plan for the corridor that's actionable, beyond perhaps the LA-Anaheim part that has potential CAHSR funding. Nobody has done the big environmental documents. Nobody has gotten beyond concept (5% or so, not even preliminary - think American HSR level of analysis) engineering for the difficult spots like San Clemente (though SANDAG somehow is still working on Del Mar). So the work needed to make LOSSAN interesting for a Brightline hasn't been done, and would be far too expensive, time-consuming, and uncertain of outcome for Brightline itself to do.