A couple reasons why. Miami has the number 1, 3-point defense in the league, depth to put players in the rotation inspite of injuries, experienced coach in Erik spolstra, veteran players who have won like Lowry and Tucker. Veteran players who have made deep runs like Butler. Familiarity with Imes coaching style since spo faced the Spurs twice in the finals already. Those are usually the hardest teams to eliminate even when you have a talent edge over them. Boston has a talent edge, but Miami has plenty of intangibles. Ironically, kinda like how that aging Boston team in 2012 had no business pushing those Heatles to 7 games. Old vets are tough to get rid of.
I'm really not seeing any intangible advantage the Heat have.
I mean, I saw their main players in Game 5 not just completely shyt the bed, but they didn't want to take jumpshots. Butler, Lowry and Bam all actively didn't want to create opportunities to shoot, and when they were open they were reluctant to shoot. It's often that core players don't shoot well, but it's extremely rate where they shy away from wanting to shoot. They were playing procrastination ball. It's why the Celtics' wins have been mostly blowouts, because those are the games where they haven't fukked around and just got down to business, and why almost all the Heat wins have been close because the Celtics have been caught playing with their food and let Miami get a foot in the door.
Honestly, there's a lot of inexplicable shyt as to why this series has gone on for this long. It's hard to put my finger on why exactly it has played out like it has. Obviously, as with every series, there are an infinite amount of factors and a logical explanation as to why we're now at this point, but it does really feel like the Basketball Gods are just fukking with us and turned this series into a Blaxploitation film.