I think one aspect is just that the decision is made definitively at some point, in advance. People have different opinions: impact on historic buildings, noise, environment, other things. This is all debated up front, and then the legislature either approves it, or it doesn't. But when it was approved, it was approved. You can't sue in court to stop the plan on environmental grounds or noise grounds or something else, once the legislature has decided to go ahead with it, because the authorizing legislation supersedes any contrary legislation.
But the U.S. delegates decision-making to agencies and courts in a way that this doesn't happen. California might take input for a long time before deciding on its plan, but its final plan is still not final. Anyone can sue it for many different reasons. Maybe it violates the federal Clean Air Act, maybe it violates property rights, maybe something else. The decision is never final until every challenge to an agency or court is decided, which massively adds to uncertainty and costs.
Another factor at play is that TGV lines are purpose built for high-speed traffic (curve radius, climb rate, ...) while for example in Germany they are mostly shared. That makes the single TGV track cheaper, but you still ahve to built another track for lets say freight. If want another track, that is.
But the cool thing ist that TGV don't stop at every single village that happens to be the hometown of some polititian.