Board Thread:Naruto Discussions/@comment-26488946-20150630094329/@comment-734582-20150630110338

The chakra of Hagoromo is present in the from of the Sharingan and the Sage body (e.g. Senju/Uzumaki). Mixing that should be enough. However, it's likely that even when the conditions are met, the rinnegan is still rare.

Consider this: The Mangekyo Sharingan is awakened by watching someone close die. We've had several Ninja Wars and even the Warring States period where hundreds if not thousands of Uchiha fought and watched people close to them die. Yet, Madara and his brother were the first to awaken this power. Sasuke and Madara only awakened the rinnegan, both with the Mangekyo. Obito had the conditions but did not awaken a rinnegan. So clearly its not an easy thing to get the Rinnegan.