The main reason that hot air balloons are only flying in the morning is because the air is the most stable in the morning and daylight hours.
It's possible to fly a hot air balloon at night but during the night the air is not as stable and could potentially cause an issue.
During the day the hot air balloon is also more visible especially to other pilots of aircraft.
So it's possible to fly a hot air balloon at night but most do not.
However in some places it can be illegal for a hot air balloon to fly at night because of visibility issues.
If you're up in the air at night and a helicopter or plane flies through and doesn't see you in the air in your hot air balloon then it can cause a deadly crash.
So it's not advisable to fly a hot air balloon during the night.
Most hot air balloons are flow during the day.
I have seen a few hot air balloons up at night but only during a festival where they likely got permission and knew no airplanes or other aircraft would be in the area during the time the hot air balloon was flying.