Why did God choose Aaron as high priest and not Moses?

I used to wonder about this. Here's an illustration:

I’ve lived in Lagos, Nigeria for most of my life. I've never stepped foot in my father's hometown, somewhere in Ondo State. The local dialect is mostly Greek to me. I have no idea what issues the people there are dealing with. I’m closer to my mom’s side of the family. She's Yoruba like my dad was, but almost in name only; very heavy Bini influence.

Just imagine how useless I would be as the chairman of the local government/senator representing the district in which my father’s village is situated. I don't even understand the local dialect. I'd be telling my advisers to “let them eat cake” very often because I'm likely to judge them and their needs based on my own personal experiences.

The high priest stood as a representative of the people to God, making atonement on their behalf. Moses grew up in the palace of Pharaoh, probably not fully aware of the travails of the Israelites in captivity and definitely did not experience any of it first-hand. No matter how many stories he was told about the suffering of his people, he never, ever felt the crack of the whip of Egyptian taskmasters on his back. Aaron on the other hand knew no other life than this one. He knew the pain of his people first-hand. He lived their suffering. He knew their weaknesses and could empathise with them. He was one of them in a way that Moses could never be.

Moses was livid when he found out about the golden calf that Aaron made for the people, just as many of us today just can't stand how some people “won't" stop fornicating, stop stealing, stop lying, stop chewing their food with their mouth a certain way, stop being so messy, try to make their marriage work, etc. But Aaron, he knew in a way that Moses could not understand how prone to sin the people were; he knew their weaknesses. He could empathise with the people. See his conversation with Moses in Exodus 32-

21 Then Moses said to Aaron, “What did this people do to you, that you have brought such great sin upon them?” 22 Aaron said, “Do not let the anger of my lord burn; you know the people yourself, that they are prone to evil. 23 For they said to me, ‘Make a god for us who will go before us; for this Moses, the man who brought us up from the land of Egypt, we do not know what has become of him.’ 24 I said to them, ‘Whoever has any gold, let them tear it off.’ So they gave it to me, and I threw it into the fire, and out came this calf.”

I'm picturing Moses’ eyes widening in disbelief at Aaron’s pleading for these people for whom God had just parted the Red Sea. Surely that, and the other miracles, how the Lord defeated the Egyptians for them, should have shown them that the Lord, He is God. Exodus 14:30-31 says-

30 Thus the Lord saved Israel that day from the hand of the Egyptians, and Israel saw the Egyptians dead on the seashore. 31 When Israel saw the great power which the Lord had used against the Egyptians, the people feared the Lord, and they believed in the Lord and in His servant Moses.”

How could they be so dumb as to go and make a god for themselves after all of that!

Jesus too had to come as Man in order to be our Great High Priest, but not like Aaron's priesthood. See Hebrews 4:14–16:

“Since then we have a great high priest who has passed through the heavens, Jesus, the Son of God, let us hold fast our confession. 15 For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin. 16 Let us then with confidence draw near to the throne of grace, that we may receive mercy and find grace to help in time of need.”

Notice receive mercy and find grace…? You receive Mercy when you escape the punishment that you very much deserve and find Grace when you get the good things that you most certainly do not deserve. None of these were available to the Israelites when they sinned. 3,000 people died. If they had attempted to draw near to the mountain in their sinful state, they would have been struck dead. But we who believe can “draw near to the throne of grace" to receive mercy and find grace in abundance in Christ.

Today, when we sin or mess things up, Jesus Christ our Great High Priest is able to sympathise with our weaknesses and deliver us from them all. He knows our weaknesses. His strength is made perfect in weakness; our weakness. As was His Way with the woman that was caught in adultery and even Zacchaeus the thieving tax collector, He will never condemn you for your sins, and not so that you should go on sinning sprees as many believe. He will save you from your sins, as it is written of Him in Matthew 1:21–

“She will bear a Son; and you shall call His name Jesus, for He will save His people from their sins.”

Moses is a picture of the law. The law cannot help anyone live right or plead on anyone's behalf. It is holy, righteous, unbending. It does not understand “I'm good in other areas but this one” or “I stole because I was hungry.” There is no sympathy or compassion shown here; no consideration whatsoever for backstories. The law demands full obedience and absolute righteousness from those who are trying to get right with God by it; and when the people fail in just one tiny area, judgment follows. Only by grace alone could Moses have been a good high priest:)

Jesus Christ our Great High Priest Himself is Grace - undeserved, unearned, unmerited favour. Aaron could not make a way of escape for the Israelites during their temptation; the arm of flesh will fail. But Jesus Christ is the Way of escape with every trial. His heart of compassion beats for us always. He knows how screwed up we can be and He helps us in our weaknesses. By His obedience, the sinner who believes is made righteous (Romans 5:19,) not so that we should continue to wallow in sins but so that we should get to live in triumph over sin and death (earthly ills are “death begun”):

“For the sin of this one man, Adam, caused death to rule over many. But even greater is God’s wonderful grace and His gift of righteousness, for all who receive it will live in triumph over sin and death through this one Man, Jesus Christ.”