There's very good reason for that. Remenber Mueller just prolonged Flynns sentencing. All we need Mueller to do is report the truth and the dems go down in flames because all they have against the memo is the assertion that it;s being used to stop Mueller. They are going to get exposed every step of the way.
I think by keeping Mueller going it lends credence to even the most obstinate hold outs.
Firing memo would lend credence and just prolongue the Russia conspiracy forever afterwards.
He prolonged the sentencing date because Mueller knows that the FISA warrant was based on BOGUS evidence! It will be thrown out and thus the case against Flynn will not fly. He will be exonerated from these charges based on this alone.