Speculations abound about why Manafort, who is 69 years old, would risk spending 25 years in jail when he could substantially lessen his sentence. One prediction is that Manafort has information that is so damning, Manafort is reluctant to reveal it. If Manafort has information that is harmful to Trump, then Mueller, presumably, is willing to make a deal with Manafort, regardless of how damaging it might also be to Manafort.
Another guess is that Manafort is afraid of being forthcoming to Mueller because the Russian mafia, to whom he owes millions of dollars, would retaliate. This explanation fails to explain why Manafort was willing to scam Mueller to get a lesser sentence: he would still have to face the Russia mafia. Manafort might find the federal witness protection program a better bet than the next 25 years in prison.
Another guess is that Manafort is refusing to tell all on Trump to obtain a Presidential pardon. Although a federal pardon would not deliver Manafort from state charges, it could offer some relief. And Mueller and his prosecutors have claimed that Manafort is trying to obtain a Presidential pardon. Manafort, however, would have to be incredibly naïve to trust Trump to deliver on a pardon. Trump could not risk the negative political backlash of a Manafort pardon until after the 2020 election. If Trump lost in 2020, he might be facing his own legal problems; if he won, he might be concerned about his legacy. If Manafort actually got out of jail, he would still be facing the Russian mafia.
One reason Manafort might not come clean with Mueller is if he an agent of Putin and the Russian government. It is in Putin’s interest to work for the release of any of his agents who are caught, to keep the loyalty of his other agents. Manafort knows this, and thus, would trust Putin. And because Putin undoubtedly has compromising material on Trump, he could force Trump to pardon Manafort. Also, Putin might be able to arrange a swap of some American agent for Manafort at some time in the future. Manafort may not be afraid of the Russians; he may be tied to them more closely than we know.