An Uchiha soul might be required to activate the sharingan, like we saw Sasuke do step by step, but I don't think that's enough alone. It must be in the Uchiha clan genes to have a sharingan or anybody could acquire it by the right mindset and training.

However, Orochimaru might be a realist in any case. If he can only use the sharingan for a time while in Sasuke's body then that's how it is, and it's the best way. Besides, while in Sasuke's body he could take a few women and have a bunch of children, one of which could have sharingan, and then when Sasuke's body wears out, he could possess his own (Sasuke's body's) Uchiha child to get a fresh and young sharingan body.