Because Hashirama and Madara were somewhat quite equals, Hashirama took permanent damage against Madara when they fought. He weakened, but still fought in the FGN War and died.
Okay here's MY THEORY Hashi and Madara somehow joined forces hashi used his wood style to make zetsu and madara used his DNA got rinnegan made Tobi, Tobi then was supposed to carry on both of their wills, refused that and killed them both, then gave the rinnegan to Nagato....Something like that Lol:ice: