Board Thread:Naruto Discussions/@comment-26488946-20150612205152/@comment-5936871-20150612223631

It's because Sasuke and Madara awakened it on their own. Nagato is an Uzumaki who had the Rinnegan implanted into his eyes as child. He didn't start off with a Sharingan later unlock Mangekyo Sharingan and then Eternal Mangekyo Sharingan and finally Rinnegan.