Why is the obligatory prayer not performed inside the Ka'bah, even though voluntary prayer is permissible there?

Question:
May Allah reward you, Your Eminence, father. This questioner says: Voluntary prayer is permissible inside the Ka'bah, so why did we exclude the obligatory prayer from this hadith?

Answer:
That the Messenger (peace and blessings be upon him) did not pray the obligatory prayer behind him. We restrict ourselves to what has been reported. Yes.