To me, it was/is a characteristic of the team when they have had a bad setback. The team had a game to potentially come back after Derek Fisher's 3 in the Laker's series but couldn't pull it off. I just think that that kind of loss (the last 30 seconds of the prior game - Miami or the Laker series) had an enormous negative confidence effect for the Spurs and an enormous positive confidence effect for the Heat. We all say that the championship was lost in that last 30 seconds, and that is why we say it, in my mind. Yes, not only did Kawhi miss the free throw (that's not really all that unusual for Spurs teams,tbh), not only did Tim miss point- blank bunnies at the end (which was VERY unusual at that time and had him slamming his hand on the floor afterwards), and not only was Manu throwing the ball around (he did it all game in that last game), but Wade had the confidence that he was going to make those shots. They believed that they could close it out and we believed that we could lose it at the end.
Then exactly that happened. Psychology is an important thing, imo.