By that time, though, you must remember he had Ani in a very vulnerable state. So, in order to keep him hooked, he holds out a bit on the promised result. He's not yet in a position where he can give Ani the equivalent of the deed to the Brooklyn Bridge and take off. He's got to keep Ani on his side at least long enough to wipe out the Jedi and take over the Republic. Plus, by that time, he's certainly figuring that Ani will be so drunk on the Dark Side that he won't care anymore about Padme, and will stay a Sith willingly. Or, if he's a liability, he gets chopped and Palpatine rules alone until he finds another apprentice. Once again, the PT is actually the story of how Palpatine bamboozles everyone.