Board Thread:Episode Discussions/@comment-1251315-20141211121709/@comment-9782468-20141213024026

His eyes crumbled because they were "fake" eyes created by Kabuto's Edo Tensei. Madara's real Rinnegan weren't with his body when he died. I guess the Rinne Tensei couldn't recreate the real deal, and the fake ones crumbled when Edo Tensei ended. Obito had one of Madara's eyes and hid the other, so Black Zetzu had to go retrieve them.