I think a big reason for what has been happening, is yes, you can say too many role players, who the Spurs need at this stage (Manu, Tim way past prime), are mentally weak. Outside the Big 3, Leonard, and Jackson, they all folded when hit the fan.
Some of the other contenders and champions didn't have this problem. Look at OKC's team, they're a confident bunch, every single player who played for them in this series. Neal, Bonner, Splitter, Green, and stretching back to previous years with RJ, they simply are not of the same mold. The Spurs role players have been great when there's no adversity, when the team is steamrolling opponents left and right because they're all collectively playing well. Once they go cold a little bit and the other team responds well, they crumble and it's contagious.