What does that have to do with the cost and timeline of the project? Why should the population density make the project take 10x as long and cost 10x as much?
You're right that it should really make it cheaper if the tracks mostly go through empty land.
Then again the fact that there isn't much need for this train due to the low potential ridership makes it so the heart of the state was never in the project, and no one was really committed to it.
They also require huge populations to travel on them to justify the huge costs. That's where California falls short.
If we doubled the population, it would be pretty OK, and we have the job market for it, but that would require constructing housing, and California doesn't do that.