Both the Cavaliers and Celtics have had large stretches this round where they simply can't get a bucket or any points in a large span (usually 4+ minutes). In today's NBA, that's insane, but it's also costly in a playoff game. It's hard to fathom given their talents. Cleveland isn't even a team that relies on the 3 as much as the C's but even they struggle at times.
The C's and Cavs have had multiple games with double digit leads and it goes away in a quick second because they can't score or pile on outside of one game (Game 3s for both)