I thought that was obvious.
Mueller is a Marine officer. POTUS has the authority to recall commissioned officers to a command out of retirement.
Mueller has been there with the deep state every step of the way. Satsuki Kiriyuin. Normally, Mueller would be compromised and unable to act. By taking good people and putting them in situations where they either have to get dirty or be destroyed, the cabal can impose a wide range of checks on those who would pose a threat.
Mueller would never be given access to the team he has been given access to under Hillary, or even of we got someone like Carson or Ron Paul. It simply never would have been as feasible without a character like Trump.
Mueller knows where to guide the teams. He can move incredibly fast because he isn't searching through large amounts of data to try and find crimes… He knows the crimes and the criminals and where to start pulling on threads to route them down.
Mueller's investigation was commissioned with an amazingly open amount of authority to pursue almost any contact with foreign governments and any crimes connected to those who are acting as foreingn agents.
Mueller was the perfect hunting dog for this. The cabal might have thought they had leverage against him. They might have believed he wouldn't dare step out of line. And they certainly couldn't sound the alarm among the Democrat body that Mueller could hang all of them. By that point, the media was already lauding him as the most noblest of noble honest noble FBI noblemen.