24hr service would be very expensive, but it seems that they should run the transit services until a little after the majority of bars and clubs close. It was nice here in Boston when they ran the T until a little past 2AM; they've since pulled that back to 12:30 or so. It can be kind of a pain, but Uber/Lyft are well established here.
It's certainly usable for that in LA, where last call is also 2am. I believe every train runs until 2 or 3 AM (on Fridays/Saturdays), and starts back up again by 5. Not quite 24 hours(except on NYE, when they don't stop), but still very usable.
It's pretty much enough to run some lines till 4am on Fridays and Saturdays, so people can go and party.
Because it's much more important to address the needs of late night partiers than nurses, convenience store clerks, and so forth who have work shifts at odd hours.