The first reason is that you fear the cross examination of your witnesses. If whatever evidence your witnesses can testify to is outweighed by the damage likely to be done by the prosecution on cross, you rest.
Second, you don't think the prosecution has met his/her burden of proof and don't think there is enough evidence to convict. Resting without putting on a single witness is a bold strategy, but it also conveys confidence to a jury, which might influence their thinking. It's like playing a hand of poker where you raise the maximum amount on every round of betting in an attempt to get everyone else to fold (even if you have nothing in your hand, it conveys to the other players that you think you've got a winner). It doesn't always work (in fact, it often doesn't), but sometimes it's your only shot. And sometimes the prosecutor really hasn't put on enough evidence to convict. Just based on what little I've heard about this trial, it certainly seems like this prosecutor has put on more than enough.
Third, to prevent damaging evidence about your client from coming out. If you're considering putting your own client on the stand in his/her own defense, you have to consider several things. Will he/she make a good witness (i.e. will he/she damage himself/herself by taking the stand)? Is your client's own record going to hurt him/her? Remember that the prosecutor can bring up your prior record if you take the stand. The prosecutor can also ask all kinds of questions that you might not want to answer, and YOU CAN'T LIE. If the answers to those questions will damage you in this trial or in any other future trial, you rest. And probably the biggest reason when it comes to this specific trial, you have to remember that this is just the warm-up for bigger trial coming up later, and whatever Manafort testifies to on the record becomes solid gold evidence against him in any future proceeding. All his admissions he makes on the stand, literally WHATEVER he says on the stand, WILL be introduced into evidence against him in any future proceeding.
My guess is that Manafort's attorneys are willing to sacrifice this "small" trial in order to prevent exposing him in the bigger trial to come. I'm thinking that his attorneys are also concerned that Manafort will bury himself with his answers to certain questions that the prosecutor will DEFINITELY bring up. Plus, he seems like the cocky, I'm-smarter-than-everyone-else type (kinda like Trump, actually) who would get up there and try to smooth-talk his way out of things and end up only burying himself deeper. So, for strategic reasons, I think his attorneys are going to treat this trial as a "preseason game" before the big one.
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