Labour costs and safety levels are going to be a huge part of this discrepancy. As are things like legal safeguards on eminent domain and environmental impact.
The real answer is much more complicated and there is no easy one-size-fits-all answer. Environmental regulation, litigious environment in the construction industry, overuse of contractors and consultants instead of in house expertise, buy america provisions, labor scheduling rules, poor planning, supplier market that has been drastically cut off due to bidding complexity and procurement regulation, bad incentives, poor oversight, corruption, incompetence, etc ad infinitum. If you had to sum it up, you could say that the US is uniquely incompetent in all aspects of government-sponsored infrastructure projects.