I have a question that why quran revelead in stages?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Allaah has explained the wisdom behind revealing the Quran in stages. He says (what means): {And [it is] a Quran which We have separated [by intervals] that you might recite it to the people over a prolonged period. And We have sent it down progressively.} [Quran 17:106]

Allaah also says in response to the disbelievers (what means): {And those who disbelieve say, "Why was the Quran not revealed to him all at once?" Thus [it is] that We may strengthen thereby your heart. And We have spaced it distinctly. And they do not come to you with an argument except that We bring you the truth and the best explanation.} [Quran 25:32-33]

As-Suyooti said in his Al-Itqaan fi ‘Uloom Al-Quran:

“If it is asked, What is the reason for the Quran being sent down in parts and not as a whole like the other (divine) books? We say: This is a question which Allaah Himself has answered, as He says: {And those who disbelieve say, "Why was the Quran not revealed to him all at once?"}, meaning as it was revealed to other Prophets before him. Allaah answered them saying: {Thus [it is]}, meaning We have sent it down at intervals so that {We may strengthen thereby your heart} meaning to make your heart stronger, because if the revelation is renewed at every incident, then this is more firm for the heart and more caring to the receiver. This requires frequent descent of the angel and him meeting (the Prophet) every now and then, along with the message that is brought to him from Allaah, and this makes him happy in a way that is impossible to describe.” [End of quote]

Also, among the benefits of the Quran being sent down in intervals is the facilitation of its memorization. Also, the abrogated and the abrogating verses occur by it, and by it there can be responses to both people's questions and incidents that take place, so the answer comes by revelation of the Quran. There is also much other wisdom.

Allaah Knows best.