you plea down to a *lesser* charge to avoid prosecution/sentencing for a greater charge (or set of charges).

we have no idea what additional charges/evidence mueller is holding on these folks. its obviously enough to compel the cooperation of michael flynn, rick gates, etc.

another wrinkle is that...once you start charging americans with collusion-related crimes...you open yourself up for discovery and have to reveal evidence/sources/etc to the defendants legal team.

we just saw this with a russian company that was charged with conspiracy by mueller. thats a pretty direct trial balloon for what would happen with american defendants.
https://www.nbcnews.com/politics/justice-department/mueller-says-russians-using-his-discovery-materials-disinformation-effort-n964811

best to hold off on the direct collusion indictments to drop all at once...or file them under seal then unseal them later.

theres been an unusual rash of sealed indictments filed in the district a mueller grand jury is impaneled in.
https://twitter.com/OpieReger/status/1092581102894473216