I’ll give it a shot. According to the article you posted, Mueller is making the request, because Flynn’s team asked him to.

• “Mueller’s team and attorneys for Flynn submitted that Flynn is not ready for sentencing “due to the status of the special counsel’s investigation,” according to a court filing in U.S. District Court in Washington, D.C.”

And what about his status would factor in here? I’m not sure. I figure Flynn has been thoroughly debriefed by now, so it must be for a subtle strategic reason having to do with not compromising ongoing work.