They pretty much chose the most expensive route possible, requiring long urban viaducts and numerous tunnels through seismically active areas. It is probably unconstructible, but the engineering consultants made out.

Vox basically did exactly this:

https://www.youtube.com/watch?v=S0dSm_ClcSw

tl;dr: US regulation makes it too easy for individuals to sue and stall megaprojects like this into nonstop litigation