I seem to remember Mueller giving the Senate permission to interview these two. For whatever reason he may be unconcerned that the interview is not going to be conducted as a high-profile public hearing. i know we'd all like to see these probable traitors swing in the wind. That they weren't subpoenaed is probably irrelevant as they agreed to testify. Maybe the senate felt that this way they could expect answers to questions that otherwise could be considered classified, and unanswerable. Though I'm not sure why either of these individuals would have access to classified information. Maybe they want them friendly for some other reason, or maybe it is just republican bullshit; but if Mueller thinks this would interfere with his investigation it won't take place.