It was a combination of things, but the primary reason was superior coaching by Pop.
His team trusts him to such an extent, that they would run into traffic if he asked them to.
There is no single player on the Spurs whose voice is recognized over Pop's.
from '86-'89 after recovering from his broken ankle, Jordan was basically the head coach of the Bulls. Doug Collins deferred every decision to Michael.
In 1990, Phil Jackson became coach and introduced the triangle-offense. Jordan controlled the practices, but only offered input to Jackson during game-time. Jackson always had the last word, and the Bulls won 6 titles in the '90's.
I can't imagine James or Wade taking Spoelstra's advice over their own.
The Heat won the championship in 2012 because their sheer physicality over-matched the Thunder's, and Brooks was as green as Spoelstra.
In 2013, they barely beat the Spurs, and did not take appropriate measures to better prepare themselves for a potential re-match. You can only blame Spoelstra for that.
On the flip side, Pop realized that it was exhaustion more than anything else, that prevented the Spurs from winning in 2013. So, he rested his star players starting from the first game of the regular season. This course of action also meant that his bench got more minutes throughout the year, thus gaining experience and confidence.
As it turned out, The Spurs' veterans were fresher, and their bench players shellacked those of Miami.
I picked the Spurs over the Heat in 4 games, and Game 2 (the game that the Heat won) was only decided in the last couple of minutes.
I love basketball, and I didn't bother to watch any of the Finals until Game 5, and I only watched the last 2 minutes of that game, just to see the confetti come down.
In my opinion, there was never a time when the 2014 Finals were riveting.