It’s a good question, one that requires a lot more of an examination that I can provide in the time taken to post this update.
But one fundamental reason: the Heat’s defense is so focused on what is happening with the guy who has the ball that they often forget about the guys who don’t have the ball. Against most teams, that’s OK because they don’t have a whole lot of weakside movement. Against the Celtics, though, that’s a problem. As bad as their offense has been this season and during the playoffs, the Celtics can always confuse teams with lots of complicated things off the ball. For a team that often loses concentration off the ball, it’s a tough matchup.