first reason is that sasuke had betetr hertigate to bigin with (uchiha sharingan n talent). naruto just cant keep up with sasuke's battle startegies. justu wise too.
in shippuuden sasuke seems to have master the sharingan, he can even use powerful genjustsu with it. remember wat mappen to naruto when itachi used a finger to make him cry. so one-on-one naruto is FUCKED.
ppl have this stupid notion that the kyuubi makes will help him against sasuke. well ppl that saud that r just dumb or have bad memory. remeber the beggining of shippuuden. sasuke can supress the kyuubi with his sharingan. so again narut ois fucked.
oh n the giant frog boss...yh even that cant help him. sasuke can summon that giant snake now. that snake fought a 2vs1 battle remmeber.
Naruto fans should just realize that n shut the hell up! dettebayo! lol