Counterintelligence investigations and prosecutions probably take a long time, and the stakes are high enough that prosecutors want to make their case airtight.

Counterintelligence investigations and prosecutions probably take a long time, and the stakes are high enough that prosecutors want to make their case airtight. Charging him with the other crimes he’s believed guilty of is probably a good way of laying the groundwork for the later case, with the added bonus of greater leverage and hurting their financial (and therefore, legal) positions.