This thread is theoretical in nature. The OP asked for an explanation that obviously has no direct answer in the databooks or manga. The only way to answer the thread is to either say "it's inconclusive" or otherwise to try to speculate based on what we know about the Rinnegan/ShaRinnegan users in the manga. I chose to attempt to answer the question, accounting for the relevant differences between the various Rinnegan users we've seen.
It's manga fact that the Rinnegan requires Hagoromo chakra, and that his chakra was in turn inherited from Kaguya. So that much adds up. It's also manga fact that Madara's third eye (RinneSharingan) only opened after he had obtained his second Rinnegan and absorbed the Shinju (although the Shinju already had a RinneSharingan). Basically, I'm comparing these examples to other manga cases where individuals had dual-Rinnegan (Nagato, Edo Madara) or Rinnegan + Shinju power (Juubito), but did not awaken ShaRinnegan. In those cases, the difference was the amount of chakra.
Sasuke's Rinnegan is obviously unique, as well. Apparently it's somehow a powered-up Rinnegan (below the level of ShaRinnegan) while its tomoe are active, since it reverts to a "regular" Rinnegan when Sasuke is low on chakra (as seen in Gaiden). Sasuke's Rinnegan can open portals to Kaguya's Dimension, and has high-level feats for a Rinnegan-user (placing each of the Bijuu under a Genjutsu in seconds, sealing all of them in Chibaku Tensei at the same time, and resisting the Infinite Tsukuyomi). It's also noteworthy that only Sasuke's Left eye was powered-up by Hagoromo; this strongly suggests that the unique Rinnegan is tied to the Hagoromo chakra being focused on Sasuke's left eye, otherwise both of the eyes would have become Rinnegan (as happened in Madara's case).
Obviously there are some questions that can't really be answered. For instance, could Hagoromo have awakened the ShaRinnegan when he was Jinchuuriki of the Juubi? Probably. But what about afterward, when he was just a Rinnegan user with Ootsutsuki chakra? Also, why couldn't Madara awaken the ShaRinnegan without his second Rinnegan implanted? He had already absorbed the Juubi and the Shinju, both of which have their own ShaRinnegan. Further, when Madara had both Rinnegan and was a Juubi Jin, why didn't they gain tomoe like Sasuke's Rinnegan? Why was Sasuke's Rinnegan unique? In fact, why didn't Hagoromo, who have Sasuke his tomoe'd Rinnegan, have tomoe in his own Rinnegan?
Obviously eating chakra fruit, in and of itself, is not the only means of awakening the ShaRinnegan. Madara never ate chakra fruit; he simply collected the different pieces of Kaguya's power. The Shinju that Madara absorbed hadn't reached a level where it was capable of producing a chakra fruit, yet it still had a ShaRinnegan. It's not as simple as saying that Shinju power = Chakra Fruit = ShaRinnegan.
Anyway, there's some of my reasoning behind the theories. I never said my answer was 100% correct and came from Kishi himself.
