Advaita Ācārya having appeared, He found the world devoid of devotional service to Śrī Kṛṣṇa because people were engrossed in material affairs. Everyone was engaged in material enjoyment, whether sinfully or virtuously. No one was interested in the transcendental service of the Lord, which can give total relief from the repetition of birth and death.
Seeing the activities of the world, the Ācārya felt compassion and began to ponder how He could act for the people’s benefit.
[Advaita Ācārya thought:] “If Śrī Kṛṣṇa were to appear as an incarnation, He Himself could preach devotion by His personal example. “In this Age of Kali there is no religion other than the chanting of the holy name of the Lord, but how in this age will the Lord appear as an incarnation? “I shall worship Kṛṣṇa in a purified state of mind. I shall constantly petition Him in humbleness.
“My name, ‘Advaita,’ will be fitting if I am able to induce Kṛṣṇa to inaugurate the movement of the chanting of the holy name.” “Śrī Kṛṣṇa, who is very affectionate toward His devotees, sells Himself to a devotee who offers Him merely a tulasī leaf and a palmful of water.”
Advaita Ācārya considered the meaning of the verse in this way: Not finding any way to repay the debt He owes to one who offers Him a tulasī leaf and water, Lord Kṛṣṇa thinks, “There is no wealth in My possession that is equal to a tulasī leaf and water.” Thus the Lord liquidates the debt by offering Himself to the devotee. Considering this, the Ācārya began worshiping the Lord.
Thinking of the lotus feet of Śrī Kṛṣṇa, He constantly offered tulasī buds in water from the Ganges. He appealed to Śrī Kṛṣṇa with loud calls and thus made it possible for Kṛṣṇa to appear. Therefore, the principal reason for Śrī Caitanya’s descent is this appeal by Advaita Ācārya. The Lord, the protector of religion, appears by the desire of His devotee.
Adi-lila, Chapter 3 Text 96-110