Originally Posted by Shem
I wouldn't say he was the only one who could, it was just prophesied that he would do it. Mace Windu would have done it if the "Chosen One" didn't interfere. When I was watching ROTS in the theater, I yelled out, "Anakin, kill the Emperor and fulfill the prophecy. That way you can save the galaxy a lot of pain and personal pain for yourself. Then you can leave the Jedi Order and live happily ever after with Padme."
I didn't yell it too loud, but the Star Wars fanatics nearby who could hear it were amused by my comment.
But anyway, the prophecy came true because Anakin destroyed the Sith, not because he was the only one who could do it, he just was the one that did do it.
Now I don't know all the latest canon and all so treat this as pure speculation but I always believed that the force could not be balanced wile the old jedi order exists. "A prophesy witch misread may be" (Yoda?). When Vader killed the Emperor the balanced was restored because the last living jedi was pure of corruption and he (Luke) could now create a new order.