He wasn't going to get much playtime with Green, Manu, Belli and Kawhi at SF-SF positions. To guarant playtime for Granger the Spurs would marginalized Green or Belli, which doesn't make sense. The Spurs also have Diaw and Kawhi for small ball line-ups. Granger would have improved the roster, but not by much. Green and Manu give some inches at the SF position, but are good for the other teams backups.
Not sure why is everyone wining about FA not signing with the Spurs. The team has as good chances to land a FA as any other team not named the Heat or the Lakers.
Diaw was a solid addition, Belli too.