I think Obito because at this point in the manga he doesn't seem to want to continue on with Madara's EOTM plan. Instead of using Outer Path to give life to Madara he might actually even use Human Path to stop his Edo. Also Obito stated it might just be better for him to become the jinchuuriki for the Juubi. Obviously Madara won't allow him to so, he has to get rid of him first.