Why was only Miriam punished with leprosy when she and Aaron rebelled against Moses?

Q. When Aaron and Miriam rebelled against Moses, why was only Miriam punished with leprosy?

“The Leprosy of Miriam,” woodcut from 1583 Bible.

The main explanation seems to be that Miriam was held more responsible because she instigated the challenge in the first place and then enlisted Aaron to support her.  There are two things in the text that show us this:
• Miriam is named before Aaron at the start of the account.  In every other place in the Bible where they are named together, including later in the same account, Aaron is mentioned first (conventionally, as the eldest brother in the family).
• The verb “speak against” is actually in the feminine singular in Hebrew:  “She spoke, Miriam, and Aaron, against Moses . . .”
Both of these things suggest, as I said, that Miriam originated the challenge and enlisted Aaron to support her, so she is held more responsible and given a greater punishment.  This shows us the fairness of divine justice.

One additional possibility to consider is that God spared Aaron specifically so that he could intercede as Israel’s high priest, approaching Moses with a renewed recognition of him as God’s representative, to ask forgiveness for Miriam’s sin and for his own:  “Please, my lord, I ask you not to hold against us the sin we have so foolishly committed.”  If Aaron had been struck with leprosy, he would not have been able to function as a priest according to the laws in Leviticus.  So God may have spared him in mercy specifically so that he could intercede for sin in this way as a priest. (It’s Moses who actually prays for Miriam’s healing).  We see that God’s very judgments are tempered with mercy, even if this sometimes makes them seem unfair.

Sparing the high priest so he could intercede for sin is a bit like the way God spared King David from direct personal punishment after he sinned by taking a census of his fighting men.  God may have spared David because the people still needed a king to rule their nation and lead their armies. When David saw the plague that was striking down the people, he recognized his own responsibility and prayed, “I have sinned; I, the shepherd, have done wrong.  These are but sheep.  What have they done?  Let your hand fall on me and my family.”  But instead of afflicting David directly at that point, God in mercy ended the plague entirely.

This is a warning to people in vital offices:  You may be spared immediate personal judgment not because you are entirely innocent, but because God still needs someone in your role and you’re not bad enough yet to be removed from it!  Aaron should not have concluded that he was less deserving of some punishment than Miriam for the same revolt, even if not the same punishment. Leaders today who “think they are standing firm” should “be careful that they don’t fall.”