My two cents about why Matt didn't testify: he wasn't part of the "Second Mile" group, and they didn't want to confuse the Jurors.
We know there are other victims, because during Weaver's interview last night they mentioned a "second group of men" that the Pros held back in case of a mistrial or possibly to refute testimony from the Defense (if they had put forth any, which they didn't).
If Jerry is put away for good till the end of his days, there's no need to drag Matt or these other guys through a trial. But there's also no reason for them to stay silent if they feared retribution from Jerry because he will probably be convicted.
If by some bizarre chance he isn't convicted, it's still good that they spoke out now so they can get protection from Jerry.