In every yuga (aeon) incarnations happen to preserve righteousness in society!
Why God incarnate?
Ice is solid, water is liquid and water vapor is gas. Basically it is one only. Water when solidified in refrigerator becomes ice. The same water, when heated sufficiently becomes steam. But the properties are entirely different. Ice is cold, solid and has a shape. Water has no shape but it occupies the shape of the container. Water can be heated or cooled. It is free flowing. The steam flows out in the atmosphere, has no shape or form and it is very light in weight. This is the mystery of nature! Likewise, the formless supreme, out of compassion for humanity, takes a form and name and comes into this world. This phenomenon is termed as ‘incarnation of god. It was formless like the steam but gets condensed due to the love and compassion towards humanity. This is the answer to the countless doubters who do not believe in a ‘formless entity taking a form!
Hence we find outward differences between the various incarnations of God. A formless entity can take any form. If the supreme had a permanent form, he could only come in that form. Hence, the supreme has full freedom to choose the time and place when he decides to incarnate. What are the circumstances that trigger the descend of supreme in human form? It is mainly the deterioration of values and morals in society. In one way, the preponderance of evils in society triggers the incarnation to come into the world. It was the atrocities of demons like Ravana and Kumbakarna that enabled the birth of Rama and his brothers. It was the evil deeds of Kamsa, Duryodhana, Sakuni and Duschasana that heralded the Krishna Avatar. The tasks of the incarnations are cut out. They embark on their missions with pre-decided goals and they achieve it surely. Jesus had come for a purpose and He achieved it fully before relinquishing His earthly career.
Likewise, almost all incarnations of God comes for specific purposes. It is protection of virtuous people, weeding out the evil mongers in society and finally revival of scriptural studies or Vedic studies among the people. Veda is eternal wisdom. It is the practice of Veda deteriorates but not the Veda per se. But during earlier eons, there were evil personalities called Demons, ogres or Asuras. In India, there is a vast list of such persons who were demonic in nature and they were ultimately destroyed by the Lord during each incarnation. But what about today? Now evil has entered into the mind of people. Almost every one has a fraction of evil in their mentality. If the Divine chooses to annihilate, none would survive baring a few holy persons. Hence the task of the incarnations in the present age is to transform the mindset of people at large. Divine incarnations themselves, follow the righteous ways, serve the society and they remain as a supreme example for other peoples to follow. Thus Jesus has shown the way by way of his behavior and teachings. He helped the downtrodden in society in many ways. He cured their ills. He fed them and clothed them. He gave sermons for the common man to follow. But few people had faith in his words. Hence there was a great conspiracy to drive him away from the public. There was a gap in the life of Jesus. None knew where he had gone. But historians write that He traveled widely and joined some monasteries for spiritual wisdom etc. On return from his travels, he actively started his preaching.
This naturally offended the high priests who were having full influence in the Kingdom. Finding that the influence of Jesus is growing day by day, they finally conspired to put Him on the Cross. Rest is history, His story. Because of the supreme sacrifice and the final ascension after three days on Easter Sunday, people started realizing the glory of Him. It is only after Jesus left the earth and ascended Heaven, Christianity started taking roots slowly. It is evident that people who preach and practice morals and righteousness are never favored in society. All his disciples faced torture and death. This is how people react to incarnations of God. But they never use their power for their own benefit. But the effect of the teachings of the incarnations lasts for thousands of years. That is why Christianity is practiced by more than two thirds of world population.
In previous eons, Rama, Krishna and other incarnations took place. They too lived their life in consonance with the norms of society. They have contributed a lot for the spiritual and moral uplift of the people. But, over a time, lethargy sets in. Hence God sends prophets, sages and saints very frequently to sustain the morality and righteousness in society. It is natural and it will go on as long as the creation exists!