The Susanoo is related somehow to the Bijuu. This is further supported by what Madara did in chapter 621. Him openin up the susanoo armor was convenient yes, to show off his intelligence, but I wonder. I s there something more to that?
Is it that susanoo can wrap itself around anything?
It felt like it was "made" to be compatible to the kyubi. Just look how kurama was able to wield the armor like a pro.
Why did the susanoo armor automatically fit without fail? The Susanoo surely didn't reject the kyubi as a host fir it's armor.
The susanoo was in essence, compatible with the bijuu.
Think about the susanoo's eyes in sasuke's earlier versions. They are directly similar to a bijuu when they transform into their tailed forms. The susanoo chakra was said to be cold, menacing, and evil.
I believe the Susanoo has a connection to the juubi. Maybe both bijuu and susanoo have a connection. Maybe susanoo's power, it's evil spirit like vibe was inherited from the ten tails. Maybe a piece of the juubi was passed on to Rikudo's oldest son in the form of chakra. This chakra is continually passed down and can only be unlocked when upgrading their eyes.