Dont FLAME please, I'm not a fanboy of sasuke or something it's just an idea, Since rasengan is basically the best form of shape manipulation for raw chakra, someone like sasuke who has good chakra control (Genjutsu), and shown to be able to manipulate both chakra natures and shape (Chidori's variations, fire style fire dragon tech, Enton Kagutsuchi, and Kirin) in addition that he's smart he would be able to learn it, don't you think? wich also made me think why didn't Kakashi teach him rasengan instead of chidori? note that rasengan is harmless to the user unlike Chidori.