The reason is that sasuke will kill him, or help in the killing him.
Sasuke has constantly been refered to as an avenger, and he still hasnt avenged the Uchiha Clan.
Yes, he "killed" Itachi and Danzo, but Obito was still at the massacre.
Therefore, Sasuke needs to get revenge on obito aswell.
Besides, Kakashi isnt going to kill the manga's main villain. He isnt as important as Naruto or Sasuke.
Most likely death for obito: Combined attack from Naruto and Sasuke in Kamuiland. Probably Chidori and Rasengan.