When they build that they did not have current technology.
Yes you need special infrastructure, but for a mega project like that would be the small issue.
In the end is a simple "what is cheaper to build and/or maintain", and surely there are cases where the the diesel wins, and could be replaced with hydrogen, if hydrogen cist will be low enough.
Electrification may even be better in the long run, but if the project cost much more and there are no money, a country may decide that something is better than nothing, and electrify on a later time