As with most things audio it is not any one thing but rather a bunch of them all together that makes the difference. Try coming home at 2AM firing your system up cold and see how it sounds. So one reason it sounds better late at night is it always sounds better after being run several hours.
All components are highly susceptible to vibration. One source of vibration is cars and trucks, wind, and even trees. Wind blows, trees move, roots transmit vibration into the ground. This all tends to die down and be less at night. Best of all is a fresh blanket of snow. So, vibration.
Then there's electricity. Pretty much everything running puts a bit of back EMF onto the line. Every wire is also an antenna bringing RFI noise into the system. A lot of RFI sources tend to be off late at night.
The validity of some of these effects is easy enough to test and verify. Vibration? Pods and springs work, and the sound in the afternoon with a calm fresh blanket of snow is at least as good as late at night if the wind is blowing. Leaving everything on and playing after you got to bed, you will hear almost all of that great late night sound the next day. So this proves the warm up effect. RFI? Disconnect a lot of wires by flipping circuit breakers. It will sound in the middle of the day about as good as late at night.