Board Thread:Versus Debates/@comment-29899821-20160317090255/@comment-734582-20160403165557

Ishi208 wrote: All I know of is that it's said in the Manga "A Sharingan that can activate Susanoo is a rarity", it's not saying A Mangekyo Sharingan that can activate Susanoo is a rarity. I don't see Tobi even implying it. Since a regular Sharingan can not activate Susanoo to begin with, it's clear that he's referring to the Mangekyou.

Secondly, being able to is a very different from actually doing. E.g. Kakashi may be able to learn Hiraishin, but clearly he didn't. Shisui may be able to awaken Susanoo, but clearly he didn't. Considering that Sasuke awoke and matured it through extreme rage, yet Shisui was clearly not gripped by the Curse of Hatred, it may simply be this difference that prevents him from using it.