Question
Why is it said at that time?
What is special about from chatzos until alot hashachar?
Answer
Hashem Judges during the different hours of the day. Before Chatzot is the time of severe judgement - Din, whereas after chatzot Hashem judges with Rachamim - pity. Therefore a good time for prayer and Selichot is after Chatzot.
Rabbi Ro'i Margalit
