Bannan is a former Bills player that Buf didn't want to re-sign when he left. He also wanted to re-locate as close as possible to Colorado, since he wants to live there permanently after he retires (former Colorado player) and most of his family is now within 2 hrs of him.
Jamal Williams is now being paid about the same, if not a little more to play his old team twice a year and not be too far from his old home in SD.
Jarvis Green has experience with the system 3-4 wise McDaniels wants in Denver and gets paid more than the Pats presumably offered while not having to play his former team (which he has no animosity toward, like Williams for instance) at most a few times every couple of years.
Don't forget they're all at or approaching 30, so they want to build something for a retirement. And that's not even factoring in that some of their wives/spouses wouldn't want to move to Buffalo Shanahan style.