This is the yuga dharma – the religion of this age Kali- yuga.
The second reason is a deeper reason and the best way to understand this is read to a class by His Holiness Gaur Govinda Swami called the “Greed of the Lord.”
Here is a summary
Lord Nrsimhadev Avatar
After Lord Nrsingadev had killed Hiranyakasipu He sat on the Throne and Praladha Maharaj had sat on His lap. When Praladha sat on the Lord’s lap, the Bhakti Mood of been a son was felt by Lord Nrsingadev. In this incarnation Lord Nrsingadev was the Father and Praladha was the son. Lord Nrsingadev wanted to experience this mood. So He decided that in his next incarnation He will take birth as a human and experiences this father and son Relationship.
“Both father and son relish this rasa, mellow. As the son sits on the father’s lap, the father relishes, and the son also relishes. The relishing is reciprocal. But the son relishes more than the father. So Nrsimhadeva developed a type of greed: “How can I sit on the lap of My father and relish that rasa?” From then on, after Nrsimhadeva, all the incarnations of the Lord accept a father and mother to fulfill that greed.”
Lord Ramachandra avatar
The Next incarnation was Lord Rama and He took the role of an perfect son. He experienced the mood that Sri Praladha had. But in this Incarnation the Lord also experienced the mood of friendhip from His close associates like Sugriva and Hanuman. The Lord wanted to experience that mood the mood of Friendship.
Lord Rama also developed greed. Vibhisana and Sugriva are Lord Rama’s friends. This means there is sakhya-rasa, friendly affection, in the pastimes of Lord Rama. But there are two types of sakhya-rasa sambhrama (friendship with awe and reverence) and visrambha (friendship as equals, without awe and reverence).
Lord Krsna Avatari
Then the Lord appeared in Original Form as Lord Sri Vrndavan Krsna. In this incarnation had the opportunity to experience the highest friendship with the cow-herd boys. Actually in Lord Krsna incarnation all the above moods was experienced in full.
“In Rama-lila only svakiya rasa is relished love with one’s own wife not parakiya rasa. Lord Rama, therefore, developed greed for the parakiya rasa. So to relish parakiya-rasa, Krsna made His own wives the wives of others. Therefore that greed which remained unfulfilled in the pastimes of Rama is fulfilled in the pastimes of Krsna.”
But then there was something that was much deeper that was not found in the other incarnations. That was the Love the Gopis of Vrndavan had for Him. And the person that exhibited that love was Srimati Radharani. This Love was beyond understanding, even for Krsna. Krnsa wanted to experiences this love of Srimati Radharani. But He could not do it in this incarnation.
ekatmanav api bhuvi pura deha bhedam gatau tau
“Although Radha and Krsna are one in one in Their indentity, They have seperated Themselves eternally. Now these two transcendental identities have again united in the form of Sri Krsna Caitanya Mahaprabhu.” ( C.C Adi 1.4)
“The first greed is this: What is the love of Radharani and how can I relish it? The second greed: What is My excellent all-attractive beauty? I can’t relish Myself. So how can I have it? And the third greed: What happiness does Radharani get by relishing My all-attractive excellent beauty? How can I have it? The desire to fulfil these three types of greeds remains unfulfilled in the pastimes of Krsna. Therefore the Lord appeared again as Sri Caitanya Mahaprabhu. The fulfilment of these types of desires isantaranga karanam, the internal cause, for Lord Caitanya’s descent.”