So why didn't Manafort put up any defense? I have a theory.

Manafort may have been told that a pardon from Trump is guaranteed so there would be no reason to continue the trial.

But why would Trump pardon him? Pardoning Manafort doesn't help Trump in any way and the political blowback might even be damaging to him. Trump doesn't do anything unless he gets something from it.
He wouldn't do it ... unless his handler told him to.

If Putin told Trump to pardon Manafort he would do it.
This would mean that Manafort has some unfinished business that Putin needs him for.
What this would be, I have no idea but it's hard to come up with another reason Manafort would roll over so easily.
So Putin could have promised Manafort a pardon from Trump for the Federal trial.

But Manafort still needs to go through the New York state trial and Trump can't pardon him in this.
The only way he could avoid prison this time would be to escape to a country that doesn't have an extradition treaty with the U.S.
So that could have been part of the deal too.
Putin would have promised that he would smuggle Manafort out of the US to Russia before the judgement in the New York trial.

I don't know why Putin would want to protect Manafort so badly, maybe he just wants to keep him from testifying to Mueller, but Manafort has rejected a plea deal and then doesn't even try to defend himself in a trial that could imprison him for life. This behavior is so bizarre that there has to be a hidden story to explain this.