you need both eyes to awaken susano'o i belive thats wat madara ment when he saidu can only awaken the sharingans full power when both eyes are togather
susano'o is rare even then
I think you mean why obito can't master susano
I mean what is the exact relationship between Susanoo and MS that only an Uchiha can master Susanoo with his MS not someone like Kakashi who has already awakened his MS?