why?because as yamato said hereYou must be registered for see linksyamato said it wasnt his true strength and he is strong enough to return sasuke without kurama ,And here is the another evidenceYou must be registered for see linksyamato says that naruto strength isnt't from the kyubbi's power and if he want to save sasuke he must do it with his own power not the kyubbi's eyes but his own.
As other people already stated, Yamato only said that to Naruto because Naruto use to lose control and Kurame would take over and attack anyone in sight including comrades. Naruto responses to positive feedback so Yamato said what he said so Naruto uses his power instead of power he can't control. Now fastfoward to after the kage summit and you'll see Naruto tell himself he needs the power of the nine tails to beat Sasuke cause he read Sasuke's heart when they collided (rasengan/chidori)why?because as yamato said hereYou must be registered for see linksyamato said it wasnt his true strength and he is strong enough to return sasuke without kurama ,And here is the another evidenceYou must be registered for see linksyamato says that naruto strength isnt't from the kyubbi's power and if he want to save sasuke he must do it with his own power not the kyubbi's eyes but his own.