There are a few reasons:
1. Sage Mode is the only thing that can hurt Obito. Perhaps, Madara needs Hashirama's DNA in order to achieve Sage Mode.
2. Hashirama's DNA can control tailed beasts. It was used to suppress all of the tailed beasts, before. Maybe Madara can use his DNA to suppress the Juubi.
3. Hashirama can use Wood Release. The Juubi is a giant tree. Maybe there is some sort of relationship between them. Maybe Madara can use Hashirama to control the Juubi's body like he controls trees.
4. Hashirama and Madara are immensely powerful. Maybe combining their power would make Madara a match for Obito. The real questions is what would they be called.