I assumed the reason was going to be poor urban planning resulting in sprawling cities which need longer routes. From a scan read, this didn't appear to be the point made in the article.
No, the issue is more bidding process shenanigans and regulatory overload from labor to environmental to local. It’s become prohibitively expensive to do large public works in the US of all sorts due to these things. It’s not just subways, it’s bridges and tunnels and all other infrastructure that would be amenable in sprawling megalopolises.
The article is about building costs per mile. If anything lower density should make construction simpler.