I don't know about everywhere else, but everywhere I have been & watched television, the ten o'clock (or nine, for those in the CST) hour is filled with the local newscast.
When they first started, I remember it bein' promoted as knowin' the news an hour sooner or something...
So, that's why FOX doesn't have network programmin' durin' that last hour.
But that's only a local thing. Back in the early 1990s in Arizona, FOX was on channel 15. Back then, channel 15 didn't have a local newscast. Instead, the 9pm (10pm for you coasties) slot was filled with reruns of Star Trek: The Next Generation.
Currently in Arizona, CW6 fills that timeslot with reruns of Friends
& The King of Queens
while My45 has reruns of Family Guy
& King of the Hill.