The Bhagavad-Gita (4.7-8) describes the reason behind God incarnating in this material world. "Whenever and wherever there is a decline in religious practice, 0 descendant of Bharata, and a predominant rise of irreligion at that time I descend Myself.To deliver the pious and to annihilate the miscreants, as well as to restablish the principles of religion, I Myself appear, millennium after millennium".

Therefore these incarnations are also known as 'scheduled incarnations'. However the Lord is also known as 'triyugi' or (One who takes incarnation in only three yugas.
The Srimad-Bhagavatam describes this as follows –
ittham nr-tiryag-ni-deva jhasavatarair
lokan vibhavayasi hamsi jagat pratipan
dharmam maha-purusa pasi yuganuvrttam
channah kalau yad abhavas tri-yugo 'tha sa tvam
"In this way, my Lord, You appear in various incarnations as a human being, an animal, a great saint, a demigod, a fish, or a tortoise, thus main taining the entire creation in different planetary systems and killing the demoniac principles. According to the age, 0 my Lord, You protect the principles of religion. In the Age of Kali, however, You do not assert Yourself as the Supreme Personality of Godhead, and therefore You are known as Triyuga, or the Lord who appears in three yugas." – Srimad-Bhagavatam 7.9.38 
So how do we reconcile this paradox? In a lecture given in Mayapur, Srila Prabhupada explained the reasons behind the scriptures calling the Lord 'channah' or a covered incarnation – Caitanya Mahaprabhu is described here as channah kalau. In the Kali-yuga He does not appear directly like other incarnations, such as Nrsimhadeva, Vamanadeva, or Lord Ramacandra. He appears as a devotee. Why? Because He is the most magnanimous avatara. People are so foolish that they could not understand Krsna. When Krsna said, sarva-dharman parityajya mam ekam samnam vraja-"Give up all varieties of religion and just surrender unto Me" they took it as: "Who is this person ordering like th at? What right does He have?" 

That is our material disease. If somebody is ordered to do something, he protests, "Who are you to order me" This is the position. God Himself, Kf~t::ta-what can He say? He orders. As the Supreme Person, the Supreme Being, He must order. He's the supreme controller. He must order. That is God. But we are so foolish that when God orders, "You do this," we take it otherwise "Oh, who is this man? He's ordering like that. Give up everything? Why shall I give Him everything? I have created so many dharmas, 'isms.' Shall I give them up? Why shall I give them up?" Therefore the same Lord Krsna came again as Caitanya Mahaprabhu.

Srila Bhaktivinode Thakura disclosed this fact in a book called Navadvipa Mahatmya. There is one very confidential topic of which I do not desire to speak but cannot refrain from speaking. Since the time when Lord Caitanya became unmanifest, the devotees have ordered that His pastimes be made manifest in His eternal abode. It is known throughout the material creation that my Lord Caitanya's incarnation is more confidential than all others. His confidential pastime s, of which the scriptures speak in a confidential way, do not appear in the minds of nondevotees. Maya-devi (illusion) has for a long time covered and hidden all the confidential scriptures connected with these pastimes. And aside from the many such unmanifested scriptures, Maya-devi, by covering the eyes of the panditas or learned scholars, always hides whatever topics appear concerning Lord Caitanya in the revealed scriptures also. Thus the deep pastimes of Lord Caitanya have become unmanifest. But Maya is very sincere in knowing the desire of the Lord, and she has taken away the deception from the eyes of the living entities. Thus the truths of Lord Caitanya are manifested in this material universe, and the hidden and confidential scriptures have very easily become manifest, dispelling the dilemmas of all the arguments of the living entit ies.
It is the most merciful Lord Nityananda who has manifested the truths of Lord Caitanya in the hearts of the living entities. By His order, Maya gives up her covering, and thus learned devotees can obtain that scriptural wealth. One whose doubt of this is undestroyed is a very unfortunate person. Why does he uselessly continue to live? When the Lord extends His mercy, all fortunate people become very happy. 0 people! Just see the symptoms of the unfortunate. They consider themselves intelligent, but do not accept the mercy of the Lord. Rather, by false arguments they fall into Maya's pit again and again. 
"0 my dear Kali-yuga living being, please come here. Give up all of your crooked ways. Just take this pure love of Lord Gauranga." Saying this, Lord Nityananda calls again and again. But these unfortunate people do not accept the invitation. Why don't people respect such unalloyed love? Considering very diligently, just see how all living entities are making so many arrangements for happiness. Some argue, and some practice yoga within this material world. For happiness one gives up household life and goes to the woods. For happiness one king fights another. For happiness a man chases women and gold. For happiness one cultivates art and science. For happiness one gives up happiness and teaches misery. For happiness one flounders in the ocean and drowns. 
Calling loudly and raising both His hands, Lord Nityananda says,"Come here, living entity. Give up your dilemmas of fruitive action and speculative knowledge. I shall give you the perfect activity for happiness, and in return I shall taken othing. There is no difficulty in this. There is no loss, and you will not be hurt in any way. Uttering the holy name, Gauranga, just dance without anxiety. There is no mistaking it. You will always feel pure joy." In this way Lord Nityananda distributes love of God. But the unfortunate, because of their sinful activities, do not want it. Anyone who even once says "Gauranga-Nitai" puts an end to the results of..! unlimited sinful activities.
(Syamananda Dasa)