For Susanoo you need Tsukuyomi and Amaterasu....Tobi has neither and he only has one MS therefore he doesn't have Susanoo.
i dont understand the facts that need Tsukuyomi and Amaterasu to activate Susanoo.. try to understand this:-
1) Sasuke awaken susanoo without using Tsukuyomi and amaterasu first... he uses amaterasu after activating the susanoo... so the fact that Tsukuyomi and amaterasu needed to activate Susanoo is rubbish... the Susanoo, amaterasu, Tsukuyomi is like the technique that can be use by MS users... those technique is more likely a package for MS users... maybe the degree of the technique power or shape is different from one to anothers... such as Madara Susanoo is more high level than Itachi and sasuke... Sasuke amaterasu is more efficient than itachi because sasuke can manipulate the susano shape to become weapons or sheilds...
2) Once again.. try to look at the facts that... Obito has many sharingan eyes... which means he can activate any of those eyes into MS anytime... so what the heck are u saying repeatedly that obito DON'T HAVE 2 MS!!!... open ur eyes... the facts that he don't have 2 MS is funny.... HA HA HA... he dont have his both eyes , yes that true... but who needs the same eyes to awaken the MS.. its just need 2 sharingan only .... doesnt matter who eyes is it.... Madara eyes... are u confirm that the eyes that madara uses now is his originals eyes... what if one is different to another... THE FACTS IS THAT MS CAN BE AWAKEN EVENTHOUGH THE EYES IS FROM DIFFERENT PEOPLE, JUST AS LONG AS IT WAS AT THE SAME PERSONS...
3) and try to thinks this... WHAT IS THE SUSANOO?? ... WHAT IT IS SUSANOO USES?..
a) As a powerfull sheilds against strong / weak attacks....
> Itachi never uses it before untill he was attacks by Sasuke Kirin... itachi uses it as the Sheilds...
> Sasuke saves his life from Raikage (ay) attacks by activating the Susanoo...
> Madara uses it when fighting the 5 kages...
b) also as a powerfull attacks...
> Itachi using it against nagato..
> sasuke using it killing Zetsu... amaterasu swords..
> madara using the susanoo swords to breaks the grounds during 5 kages battle...
# but most importantly... look closely... those people who activate the Susanoo... what is the similarity... they vulnerable ( expose) to attacks if they not using the Susanoo... those poeple can be touch.. any strike / attacks can hit them ... so , to sheilds themself.. Susanoo is the best tools...
As for obito... he didnt need to activate the susanoo YET!!! because no one can land an attacks of him... If his live was threatened , Im positively sure that he will using the Susanoo