It's pretty simple. Firstly, the Cavs are better. Period.
Secondly, the Cavs seven players who shoot league-average or better from 3. All of them are healthy. The C's have 4, two of whom were hurt. (Arguably we had five, since injured Crowder pulled his numbers below league average, but he was still hurt during the playoffs.). It's a lot easier to play defense when you can ignore several players on the perimeter.
Thirdly, they have LeBron, who is very, very good, especially when on a team that requires defenders to stay home or give a good shooter an open 3, which, as already noted, the Cavs are.
I will always be convinced that a healthy Celtics team beats a healthy Hawks team this postseason, but that's not what we were. It's been no question that we were limited shooting from the perimeter. And while in one sense we had depth, it was more defensive depth than shooting depth. Of course, Ainge knows this. He knew we weren't that close, which is why a trade wasn't made for Horford, for example. The series showed us nothing new, except maybe some of us, myself included, how we need to have multiple scoring threats on the court at the same time, because good defenses can take away the main option, as we saw with IT at the end of the series. But we also have a solid foundation, that needs that one great player to join on and take it to the next level. Luckily we have the cap room, players, and picks that give us a great chance to get that player. Ainge will keep swinging for the fences, all the while improving incrementally if the homerun isn't there.