Because the IRS depends in a large part on voluntary taxpayer compliance and being truthful on your tax returns.  Tax returns have absolute privacy as far as the IRS is concerned and cannot be disclosed to anyone without the taxpayer's permission.  Knowing that an IRS employee could breach this trust and then get a slap on the wrist (or a parade in the case of idiot Joe) would erode public confidence that their tax returns are protected against this sort of thing by the IRS.  Even though the returns turned out to be a nothing-burger and fake news media quickly moved on, the public confidence in the sanctity of their privacy rights is the greater concern.  People will tend to not be as forthcoming in their self-reporting if they think it can be handed over to anyone else.  This is why he was made an example of, so other IRS employees will follow the law in protecting taxpayers' privacy.  

The judge is a foreign born Biden appointee who was confirmed by the Senate on a party line vote.

sacrificing one of their own,  a  low level schlub, to make it look good.  

 
Let's Go Brandon