They want make sure the case is ironclad.

it might be someone who participated in the conspiracy, but might be helping the prosecution in exchange for favorable treatment. This, of course, is not The Pustule, Don Snoreleone.

The DOJ's manual for prosecutors says not to do it because it is a violation of the 5th amendment to the Constitution.

9-11.130 - Limitation on Naming Persons as Unindicted Co-Conspirators

In the absence of some significant justification, federal prosecutors generally should not identify unindicted co-conspirators in conspiracy indictments. The practice of naming individuals as unindicted co-conspirators in an indictment charging a criminal conspiracy has been severely criticized in United States v. Briggs, 514 F.2d 794 (5th Cir. 1975).

https://www.justice.gov/jm/jm-9-11000-grand-jury#9-11.130

the indicted folks could spew about the clown to get a better deal. And then the law will have more on the clown when they do indict him