Factors:
People's labor is no longer so cheap to build stuff, they have more qualifications and alternatives to their labor and they want to be paid more.
Government contracting becomes influenced by political / additional considerations.
The projects get more and more complex and are no longer decided by just a few responsible people but dispersed among multiple agencies each with requirements.
Regulations and standards increase.
There are more people to insert their hands into the revenue streams and extract some value.
Property now is worth something and has to be bought off.
It goes on and on. I don't know if there are any good examples of where an advanced economy was able to avoid this increasing cost of basic infrastructure building, although of course, some countries manage to do it without the same levels of cost as the USA for example.