Originally Posted by Emissary of Justice
The bigger question: Why didn't they just absorb it using the Rinnegan?
To answer your question, it's because Obito and Madara were more or less focusing most of their power to control the Juubi. Just like how Obito couldn't do much when he had the 6 jinks under his control. If either of them lose focus or give the Juubi too much power, they'll lose control of the beast.
Another reason why Nagato can't use the rinnegan to it's fullest because of the amount of chakra being used to control the 6 paths of Pain.