It is really that simple. Like Doc said after the game, it will always be a make-or-miss league. You can have the perfect offensive execution, get a wide open shot, miss it, and the other team can throw in a buzzer-beater off the backboard.
In this case, the C's lost because a career 30% 3-point shooter hit a contested, corner 3 in front of the C's bench in Game 4, then followed that up with a 5-for-7, Ray Allen-like 3-point barrage in Game 5, including 2 fade-away 3's with Pierce draped all over him with the shotclock winding down.
Let's recap Doc's make-or-miss statement:
Game 4 - Celtics up by 3 after two consecutive 3-pointers by guys who actually make these shots. Crowd is going crazy. Shot clock is winding, down. Pierce is up in Lebron's face, with the whole Celtic bench screaming at him. He hits a fallback 3 from the corner...crowd goes silent. Still, game is only tied. Celtics then get a wide open layup opportunity from Rondo - he misses it. Game still tied. Celtics then get a last shot in regulation, screw up the play, Pierce misses. OT, turnovers plus a ridiculous fade-away buzzer-beater by Wade does the Celtics in.
Game 5 - Not 100% sure of this sequence, but...Ray misses wide open 3 (rims in and out), Lebron hits fadeaway three with shot clock running out and Pierce in his face, Jeff Green rims out tough but makeable running layup in paint, Lebron hits fadeaway three with shot clock running out. Heat up 6 with 40 secs left. Game over after turnover on next possession.
If Lebron James hits 5-of-7 threes in any game, Miami will not lose. Ever.