I feel like I'm the only one that reads the manga now... The reason it's in only his left eye is because of the overwhelming amount of yin from hagoromo plus indra's chakra. This lead to the formation of an Mangenkyou Rinnegan if you will, it's the closest eye to Kaguya's eye as there is only one version of it. Indra and ashura's chakras are just watered down versions of hagoromo's strength, hence why by mixing their chakra you only get the regular rinnegan, to Sasuke, a regular rinnegan eye, as shown in the manga is a depowered state, furthering the notion of supremacy of a one eyed Tomoe rinnegan being more potent than a regular rinnegan as not only are it's rinnegan powers seemingly better used as tomoes (if not on par) but they also retain his sharnningan powers, giving him an edge. Which would explain why in gaaiden sasuke couldn't use ameterasu while using a tomoeless rinnegan as the left eye is the eye that casts Ameterasu in the first place and regular rinnegan does not retain past sharnningan properties. Tomoe rinnegan retains past sharnningan properties but seemingly heightened, for example. Sasuke could use rinnegan:genjutsu as opposed to sharnningan genjutsu, also you could argue sasuke prediction capabilities are heightened while being a Tomoe rinnegan user as the average sharnningan user would not be able to keep up with the speed of RSM users while a Tomoe rinnegan user would be. Also the copy capabilities are there since it retains sharnningan properties so that would explain how Sasuke was able to utilize rasengan variants, though he could do this with sharnningan, rinnegan makes it more potent. Does this clear up confusion?
