Board Thread:Naruto Discussions/@comment-31948035-20170603210118/@comment-30323490-20170714073136

Because Obito is a TTJ and it's pretty hard to get rid of him. Either you have a strong enough seal like the SP-Chibaku Tensei or extract the tailed beasts from him. The first one, Sasuke doesn't have, and it's debatable if Sasuke can extract the Bijuu all by himself, but not in his favor, considering the amount of people (and the people themselves) that was needed to do it in the first place. If he didn't have that TTJ immortality, Sasuke wouldn't have that much hard time to deal with him.