Must I repeat this again? He's a Ninjutsu based character. And Ninjutsu doesn't work on Obito.
The only way Sasuke could hit Obito is if he somehow get's the curse mark back from Orochimaru, or Juugo gives him some natural energy through his curse mark.
The Curse Mark is an unbalanced way of channeling natural energy. Since the body doesn't know how to handle it right, it eats away at the cells and it also causes the body to take on a different form...even in some cases making the user go crazy, like Juugo does when he goes into final curse mark mode. The only way Sasuke can somewhat control it is through his sharingan which notably uses up a lot of chakra.
- When Sasuke lost control of Orochimaru in front of Itachi, his entire curse mark seal unleashed Orochimaru and it was gone after that...his sharingan also deactivated.
So technically Sasuke had a form of Sage mode back in Part 1 when he faced Naruto at the Falls. Cursed Mark Seals given by Orochimaru are then technically incomplete forms of Sage Mode. It's a way of accessing natural energy.