The following passage from Srila Prabhupada's Srimad-Bhagavatam introduction describes how and why Lord Sri Caitanya Mahaprabhu began preaching his most sublime sankirtana movement.
After taking sannyasa, the Lord made His headquarters at Jagannatha Puri due to the request of His mother, Srimati Sacidevi. The Lord remained for twenty-four years at Puri. For six years of this time He traveled continuously all over India (and especially throughout southern India) preaching the Srimad-Bhagavatam.
Lord Caitanya not only preached the Srimad-Bhagavatam but propagated the teachings of the Bhagavad-gita as well in the most practical way. In the Bhagavad-gita Lord Sri Krsna is depicted as the Absolute Personality of Godhead, and His last teachings in that great book of transcendental knowledge instruct that one should give up all other modes of religious activities and accept Him (Lord Sri Krsna) as the only worshipable Lord. The Lord then assured that all His devotees would be protected from all sorts of sinful acts and that for them there would be no cause for anxiety.
Unfortunately, despite Lord Sri Krsna's direct order and the teachings of the Bhagavad-gita, less intelligent people misunderstand Him to be nothing but a great historical personality, and thus they cannot accept Him as the original Personality of Godhead. Such men with a poor fund of knowledge are misled by many nondevotees. Thus the teachings of the Bhagavad-gita were misinterpreted even by great scholars. After the disappearance of Lord Sri Krsna there were hundreds of commentaries on the Bhagavad-gita by many erudite scholars, and almost every one of them was motivated by self-interest. Gaura Purnima
Lord Sri Caitanya Mahaprabhu is the selfsame Lord Sri Krsna. This time, however, He appeared as a great devotee of the Lord in order to preach to the people in general, as well as to religionists and philosophers, about the transcendental position of Sri Krsna, the primeval Lord and the cause of all causes. The essence of His preaching is that Lord Sri Krsna, who appeared at Vrajabhumi (Vrndavana) as the son of the King of Vraja (Nanda Maharaja), is the Supreme Personality of Godhead and is therefore worshipable by all. Vrndavana-dhama is nondifferent from the Lord because the name, fame, form and place where the Lord manifests Himself are all identical with the Lord as absolute knowledge. Therefore Vrndavana-dhama is as worshipable as the Lord. The highest form of transcendental worship of the Lord was exhibited by the damsels of Vrajabhumi in the form of pure affection for the Lord, and Lord Sri Caitanya Mahaprabhu recommends this process as the most excellent mode of worship. He accepts the Srimad-Bhagavata Purana as the spotless literature for understanding the Lord, and He preaches that the ultimate goal of life for all human beings is to attain the stage of prema, or love of God.
Many devotees of Lord Caitanya like Srila Vrndavana dasa Thakura, Sri Locana dasa Thakura, Srila Krsnadasa Kaviraja Gosvami, Sri Kavikarnapura, Sri Prabodhananda Sarasvati, Sri Rupa Gosvami, Sri Sanatana Gosvami, Sri Raghunatha Bhatta Gosvami, Sri Jiva Gosvami, Sri Gopala Bhatta Gosvami, Sri Raghunatha dasa Gosvami and in this latter age within two hundred years, Sri Visvanatha Cakravarti, Sri Baladeva Vidyabhusana, Sri Syamananda Gosvami, Sri Narottama dasa Thakura, Sri Bhaktivinoda Thakura and at last Sri Bhaktisiddhanta Sarasvati Thakura (our spiritual master) and many other great and renowned scholars and devotees of the Lord have prepared voluminous books and literatures on the life and precepts of the Lord. Such literatures are all based on the sastras like the Vedas, Puranas, Upanisads, Ramayana, Mahabharata and other histories and authentic literatures approved by the recognized acaryas. They are unique in composition and unrivaled in presentation, and they are full of transcendental knowledge. Unfortunately the people of the world are still ignorant of them, but when these literatures, which are mostly in Sanskrit and Bengali, come to light the world and when they are presented before thinking people, then India's glory and the message of love will overflood this morbid world, which is vainly searching after peace and prosperity by various illusory methods not approved by the acaryas in the chain of disciplic succession.
SB Introduction