The reason hot air balloons usually launch during the early morning hours is because the air during those times is usually the most stable.
For a hot air balloon to take off into the air the air surrounding the balloon needs to be stable and any instability in the air and weather can mess with a hot air balloons flight.
So it's important to have good quality stable air for the hot air balloon to take off into the air.
The weather temperature during the early morning hours is not as hot so it takes less heat to get the hot air balloon into the air as well.