**Why didn’t God send the Qur’an during the time of Musa? Why wait for Mohammad’s birth? **
Because Muhammad (s) came into existance before Musa (a). His birth as we know it was in 570AD. As the qur’an states “They know only what is aparent of the life of the world”. But his existance before bith is another story all together.
Narrated Muhammad ibn Ziyad:
Ibn Mahran asked Abdullah ibn Abbas about the meaning of the the verses "And most surely we are they who draw themselves out in ranks, And we are most surely they who declare the glory (of Allah). Qur’an 37:165-166
Ibn Abbas says, we were in the company of the messenger of Allah (s) and Ali ibn Abi talib approached, when the messenger saw him a smile appeard on his face and he said “greetings to the one whome God created before his father Adam (as) by 40,000 years”
Ibn Abbas asks, Oh messenger of Allah, is it possible the son to be before the father? The messenger answered Yes, Allah created me and Ali before creating Adam by this length of time (40,000 years).
He created devine light and split it into two. Created me from one half and Ali from the other half before creating anything else. Thus the light of all the creations is from my and Alis light.
Then He placed us to the left of the throne. Then He created the angels. So we declared His glory then the angels also declared his glory. We declared His greatness then the angels also declared His greatness.
This (action of praising God) was from my and Ali’s teaching.
It was in God’s knowledge that the angels would learn from us how to praise and glorify him, and everything else that praises God is from our teaching.
It is also in God’s knowledge that He would not punish with the fire those who love me and Ali. And he would not allow into heaven those who hate me and Ali.
Biharul Anwar Vol.36 Chapter 8 Hadith #18
They taught us how to worship our Lord and for this it is of every ones duty, Angels, prophets, Moses and Jesus (as) included to celebrate the memory of the Prophet Muhammad (s).
God decided the matter to take place in this order, the last and final message to be delivered by the first and greatest creation.