Board Thread:Naruto Discussions/@comment-3232093-20140617193601/@comment-1512913-20140821171740

It's much simpler. You add chakra to the Sharingan in order to upgrade it. Kaguya was the first one with chakra, all chakra for that matter, so there was nothing for her to upgrade, she got the One-eyed tomoe Rinnegan from the get go.