I think it was a combination of a few things. First, the injuries throughout the season to TD and Manu really hurt them. They just never seemed to get the team into a steady flow. Tim was finally starting to come around in the playoffs but Manu was still very inconsistent.
Secondly, the Spurs seemed to let Dallas dictate the style of the games moreso than any other series I've seen them play. Yes, the Spurs ran with the Suns last year but at the home games they tended to play more the Spurs style. There weren't really any games in the Dallas series that looked like the Spurs imposing their will.
Finally, I think there were just a few unlucky bounces and some slips in the concentration of the players. Let's face it, with the exception of Game 2, the games were extremely close. A play going a different way here or there means a completely different outcome.