Something unreliable is useless - if it stops the train when there's no need, it will cause delays and annoy passengers. It won't be trusted and the drivers' unions will ensure any fault in driving detected with it had no disciplinary effect. But the politicians won't want to spend more on a useful system after buying GPS.

A better question is why didn't the USA adopt European train control systems. Those are well regarded, and many countries buy them. Is it a lack of budget, political will, or "not invented here"?