Quran did not come down from heaven to Prophet Muhammad (peace and blessings of Allah be upon him) all at once; rather the Revelation came down to him from Allah piecemeal, according to events and circumstances.

Allah, may He be exalted, says in Surah al Furqan Chapter 25 Verse 32:

"And those who disbelieve say: Why is not the Quran revealed to him all at once? Thus (it is sent down in parts), that We may strengthen your heart thereby. And We have revealed it to you gradually, in stages."

The fact that the Quran was revealed when new developments occurred meant that it had greater impact and was more reassuring to the Prophet (peace and blessings of Allah be upon him). And that was more effective than if it had been revealed before that, then was remembered when the new developments occurred.

Allah, may He be exalted, says in Surah al Israa Chapter 17 Verse 106:

"And (it is) a Quran which We have divided (into parts), in order that you might recite it to men at intervals. And We have revealed it by stages."

Scholars say: "At intervals" means at an easy pace, so that they might ponder it and think about its meanings, and derive knowledge from it.

And We have revealed it by stages," means: bit by bit, piecemeal over twenty-three years.