The Reason why the Qur'an was revealed in Stages, the Refutation of the Disbelievers, and their Evil
Allah tells us about the many objections raised by the disbelievers, their stubbornness, and how they spoke of things which were none of their concern. They said:
("Why is not the Qur'an revealed to him all at once") meaning, why was this Qur'an, which was revealed to him, not sent down all at one time, as the previous Books, the Tawrah, Injil, Zabur and other Divine Books Allah answered them, telling them that it was revealed in stages over twenty-three years, according to events and circumstances, and whatever rulings were needed, in order to strengthen the hearts of the believers, as He says:
Verse no. 33 supports and accentuates the explanation given above for the wisdom in the gradual revelation of Qur'an in as much as it promises Allah's support in any situation of awkward questions asked by the infidel.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 31–33
Whenever a preacher of the Truth calls for its acceptance, those who represent untruth in the name of Truth, become his enemies. They raise a variety of objections and try to cast doubt upon his veracity. In so doing, they are successful in winning wide support. On the Day of Judgement, those who, taken in by the assertions of false leaders, and not having co-operated with the preacher of Truth, will clearly see that the arguments of their leaders had no real basis in fact. They were simply false objections which they had accepted, as they suited their self-interests, and they had used them as excuses for distancing themselves from the Truth. On the Day of Reckoning they will regret having been taken in by the false arguments of their leaders and not supporting the preacher of God’s word.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.