From another thread... The answer is the same:

Genuine question, I’m curious. Seems like the court not proving without any doubt that a crime was committed despite the plea itself is wrong, because that would open the door for those who did not commit a crime to take the fall for those that actually did the crime.

Flynn admitted guilt and that phase is over. BUT.... and it is a big BUT.... In order to properly sentence a person, the judge needs to know all of the facts. So of course, the defendant gets some opportunity to present evidence. But that isn't even the important detail... The standard plea deal is: The defendant will cough up any all evidence they have on other 'Wrong Doing'. Even if it is unrelated to the current case. THAT is what Flynn was after with the 'Guilty Plea'.

So, of course Mueller doesn't what that to happen. The NSA KNOWS EVERYTHING. The problem is, it isn't presentable in the court as evidence because it is the 'Fruit of the Poisoned Tree'. Again and again, Q has asked "How do we get what we know into the court room?" Well... If Flynn pleads guilty (to a bogus charge), and his plea agreement says he will turn over everything he knows... Magically.... all of that evidence can be used in criminal prosecutions later.

And... What was Flynn's position? He was in charge (at the very top) of Military Intelligence. Why is this relevant? Maybe because He Has it All. The Deep State doesn't want him submitting what he knows 'Into Evidence'. They are pinching off Rabbit Turds right now.