Unless you'd like to go from Boston to DC without making four stops along the way. In California you have two of the largest metropolitan areas in the country separated by 400 miles of nothing (with apologies to the Central Valley). HSR wins big when it goes fast without stopping, which is exactly the case out here.

NYC CSA is bigger than LA CSA, and DC CSA is bigger than "Bay Area" CSA. It seems weird, then, to argue it makes more sense to connect LA and SF just because in the northeast you can also hit two additional CSA's (Philadelphia and Boston, which are each about the size of the Bay Area), while you're at it.