>>283102410
Have you tried reading the manga? Once Orochimaru gets a hold of a body, all of its aspects kinda keep staying with him even after he does the next switchiroo. He wanted Sasuke's body because sharingan can only reach its true potential when uchiha blood is flowing through your veins. Kakashi would never reach the true mastery of sharingan a genuine uchiha is capable of. Being out of it for several weeks straight right after using it was one of the consequences of it