It's not that hard to figure out, actually.  The Spurs is probably the most adaptable team in the league, and can adjust their performance to suit their opponent.

But the Suns only know how to play one way:  run and gun.  Sure, Stoudamire can go buckwild on offense, but what is he going to do on defense?  Nothing.  Steve Nash has never been a defensive threat (his defense is about as easy as walking through a bubble), and thus never provided a threat to either Tony Parker or Manu Ginobili.

In other words, Steve Nash left the old Dallas in order to go to the new Dallas.  And just like the old Dallas, the new Dallas will watch the NBA Finals either at home or in the stands.  Meanwhile, the Spurs can take eight days off while they await Detroit or Miami.  That's what happens when teams try to play on only one end of the court.