Because as a secret reveal it's not great... I looked at the film, an a lot of the press around it, and thought 'Well, they've blown that one a bit early, haven't they? I can't beleive they still think it;s going to be a surprising reveal..." So then I ruminated on the idea that they were just tricking everyone into thinking the two were the same, and the Ian McDiarmind connection was a matter of cloning or something... which at leasts answers all those questions about how Palpatine remained unnoticed as a Sith Master...mind you, you can make up any reason you like for that one.
But the more time goes by the more it just looks like a rubbish secret, that they can't even be botheresd to keep secret. Oh well.