Mueller’s team is largely investigators who are experienced in financial crimes. If Manafort engaged in money laundering and these guys have unravelled the layers of transfers to various entities, they’ve got him dead to rights, and it would be malfeasant to negotiate a solid case of financial crime that they could prove against testimony that likely would amount to nothing on an unrelated matter (i.e. “Russian collusion”).
Here’s an analogy: Suppose they’d found child porn on his computer while looking for evidence of collusion. There is no doubt they would have charge him with that, even though it was incidental to the collusion investigation. And citizens who don’t believe anyone should be above the law, that’s the way it should be.
As I understand the case, these are from activities back in 2005. Statue of limitations is 6 years from the later of commission or discovery. Critical to the time line is when the FBI investigated and decided not to prosecute. If the case was started prior to 2012, then the statue of limitations has already run out.