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). So they were led indirectly to awaken the rinnegan. The others didn`t know about the combination. The only reason why Madara took a chunck off Hashirama at VOTE was because he know he had to combine their chakras in order to obtain something greater and more powerful, which turned out to be the rinnegan. Sharingan is not required for rinnegan.