On 16th August 2017, the second leg of the Spanish Supercopa was played. The starting time of the match was 11 PM (UTC +2).
According to ¿Por qué el clásico de Supercopa se juega a las 23:00? (Why does the clásico of Supercopa is played at 11 PM), they say (my translation from Spanish):
Because that day there are Champions League calification matches, which are played at the normal time 20:45, and UEFA imposes no official matches to be played while their own official competitions are being played. So Spanish Supercopa could not be played between 8.45 PM and 10.30 PM.
However, what is the reasoning for UEFA to have this rule? Since they are completely different competitions, what is the problem on having them on the same time?