Most likely, Trump didn't want to be accused (falsely) of obstruction. So he leaves RR alone until Mueller is finished. That's all.
[RR] was supposed to be "their" guy.. Check out his approved vote.. Almost unanimous.. He give Barr and POTUS insulation from bias. They are including [RR] in all the decisions. "They" approved of him so they can't batch about what's he's been party to.