IMO he was let go because the intelligence community and GOPe (Pence, Priebus, Ryan, etc.) went after Flynn in part to keep Trump from dumping Priebus.
That the WH now has “full confidence” in Priebus is bunk. Either Trump and Bannon are stupid, which they aren’t, or they are now captive. Pence was the trump card that Priebus and Ryan could pull on Trump—and they pulled it.
https://youtu.be/fnjHVuC7sjU