There's a good site with multiple studies on this question: https://transitcosts.com/
Basically there are a lot of reasons, but a few key ones are:
- The US doesn't have centralized organizations for planning and designing rail systems. We mostly have small regional systems and each stretch of rail and each station is bespoke requiring an outside (expensive) company to design and built it.
- Bidding process along with accountability is a bit broken. Contractors aren't required to show line items and penalties for overruns and delays cause them to submit higher bids. The desgin/build process is also poor in part due to the previous point.
- Environmental restrictions cause massive delays, don't have proper coordination to expedite, and are easy for people against a project (or even sometimes for) to submit complaints that cause everything to become later and more expensive.