Board Thread:Naruto Discussions/@comment-36221354-20180717114536/@comment-30323490-20180717200035

Because there were only 2 people who awakened the rinnegan after Hagoromo: Madara and Sasuke. They were both Indra`s reicarnation, and they had the biggest chance of awakening the rinnegan since they already had half of Hagormo`s chakra and they only had to get the second part (Ashura`s).You can argue that Ashura reicarnations had equal chances, but for instances the Uchihas were giving hints in time that they need to combine their power with Asura`s reicarnations (since they had the Uchiha`s stone tablet which gave them hints).

Sharingan is not required for rinnegan.