Which subway are you referring to?  On which city?.  In Beijing, for example, the one I am aware of, the trains roll from 5 am until just after 11 pm, and that is leaving the first station.  Effectively, they roll from 5 am until almost midninght.  And on specil events or festivity (holiday) days, they run even later at night.  My guess is that they stop for lack of passengers during normal sleeping hours.

I dunno - other than logic... lack of paying passengers equals a loss of money. How much does it cost to run a subway? How much will you get with half empty carriages?

In the capital cities in Australia, ours public transport has buses/trains ('train' probably is what you call a 'subway' or 'metro') up to 2am on weekends to most large suburban hubs... that's not including special events like NYE.

Because you can't get power from the moon

They close down for maintenance and to inspect the infrastructure. In Australia inspections and minor maintenance are done in the evenings in a 3-4 hour window while major component replacement (rail,turnouts,rail grinding etc) is done when the system closes down for a weekend on 3 or 4 occasions each year. In China they never close the system for days as it is too busy so they need longer windows to replace components progressively during the evening.

I also guess they they really don't care how people get home after a night out. They have taxis for that.