Board Thread:Naruto Discussions/@comment-25294339-20141101162145/@comment-4685024-20141102044041

I say the most logical explanation for Obito reviving Madara has is that Obito needed to fully rebuild Madara's body and reattach his soul to it, which is much more complicated and taxing than just summoning a soul to return to it's body.

Madara's soul was also already there, so the King of Hell would not need to used and would also explain how he could revive a long deceased person, if the main issue with reviving a long dead person was fetching their soul from the other side.

With that said, it also explains why Nagato couldn't revive Yahiko, since his soul was beyond reach. However, if Yahiko was brought back with the Edo Tensei, then Nagato would have access to his soul and be able to put it back into it's original body. Hypothetically speaking of course.