I'm guessing that Atlanta figures to let Jamal Crawford walk. As Crawford's replacement, McGrady gets to jack up as many shots as he wants to every night off the bench. Atlanta is good enough to make the playoffs in the East, so he gets to play on a "winner" as well.
Little expectations/little pressure for McGrady in Atlanta. In Chicago, he'd have to fit into the starting lineup (defer to Rose) and deal with a lot more scrutiny/pressure.