Tracks need maintenance, so they can't run 24 hours. NYC is an exception - also why I already said it's great - because most lines have 4 tracks. They can use two and run maintenance on the other.

BTW, here's a thought. The Hong Kong MTR opened in 1979 (I rode it on the first day). Except for the newer cars (which aren't much different from the originals), platform doors and Octopus, most aspects remain the same. So, can we say that subway technology has basically reached a plateau worldwide? That's unlike regular trains, where there are so many more high-speed rails around the world since 1979.