Why did the Messenger, sallallaahu ‘alayhi wa sallam, say to Bilal on the day of the Conquest of Makkah, "Call the azan," instead of doing it himself?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

He, sallallaahu ‘alayhi wa sallam, did not do this himself for a number of apparent wisdoms, and all knowledge is with Allah, the Exalted.

Firstly: His Sunnah is that the imam is not assigned to call the azan (rather, there is a muezzin for that).

Secondly: This caused the polytheists to be enraged, seeing Bilal, the one upon whom they used to inflict various kinds of torture for being a Muslim, rising over and in front of them atop the Ka'bah to call the azan – the main symbol of Islam – while he is the most honored and they are the most humiliated. This is strengthened by what Ibn Katheer mentioned in Al-Bidayah wan-Nihayah by way of Yoonus ibn Bakr and others from Hisham ibn ‘Urwah from his father, that the Messenger of Allah, sallallaahu ‘alayhi wa sallam, ordered Bilal to call the azan on top of the Ka‘bah on the day of the Conquest in order to infuriate the polytheists.

Thirdly: That act was an honor for Bilal, who endured great harm in the cause of Allah. Ibn Katheer mentioned in Al-Bidayah wan-Nihayah from Ibn Abi Maleekah:

The Messenger of Allah, sallallaahu ‘alayhi wa sallam, ordered Bilal to call the azan on the day of the Conquest on top of the Ka'bah, so a man from Quraysh said to Al-Harith ibn Hisham, "Do you not see unto where this slave has ascended?" He said, "Leave him, for if Allah dislikes it, He will change it."

Allah knows best.