I don't think it's "can't" but the Spurs should be strong underdogs in that matchup. I'd say the Thunder would have about an 80% chance of beating the Spurs as things stand.
1. Durant is by far the best player on either team. In the NBA, that matters a ton.
2. Their stars can play huge minutes. , Durant can (and did) play the whole game, same with Westbrook. Compare that to the Spurs, whose stars need their minutes limited. Even in the playoffs, TD goes about 35, TP about 38 and Ginobili about 30. That hurts when Durant goes 48 and Westbrook goes 45.
3. The Spurs rely on passing. The Thunder are loooooooooooooong and make passing much more difficult than it is against any other team.
4. Trapping Parker with Sefolosha and another long player with Ibaka waiting to block the shot under the rim is the most difficult minefield TP has to deal with in the league.
5. The Spurs by design allow opposing bigs to shoot ~18 footers at will. It's the one area they basically forfeit in their defensive scheme. Unfortunately, Ibaka and Collison (and even freakin' Perkins last playoffs) can hit that shot all day now.
6. When it comes down to it, the Spurs really don't have an ace perimeter defender to throw at Durant. Parker does great on Westbrook but the Spurs don't have a SG/SF who is even routinely above average.
7. A lot of San Antonio's defensive philosophy revolves around the Spurs not fouling in order to keep the opponent off the line. For whatever reasons, that strategy doesn't work against OKC. No matter what the Spurs do, the Thunder will get to the like 25-35 times per game.
Oh, and the chances of the Spurs even making it to the Thunder 100% healthy are pretty long. The Thunder literally never suffer an injury while the Spurs deal with a new injury every couple days.
The Thunder are just a bad matchup for the Spurs. What sucks is that the Spurs actually matchup pretty damn well against the Heat. San Antonio is great at neutralizing drivers (like LeBron and Wade) plus the Heat aren't nearly as athletic as the Thunder (the Spurs might actually be more athletic due to the number of washed up scrubs on Miami's roster).
I wouldn't be surprised if all things equal:
Spurs > Heat
Thunder > Spurs
Heat > Thunder
... which of course would lead to the Heat winning the championship again.