Silas made himself appear as other people so that no one would know who he was and when he was coming. Then, he appeared as a disfigured monster to Bonnie, because he wanted her to believe that was his true face and feel sorry for him and his sob story. My best guess would be that he was waiting to reveal his true face to Stefan, because the writers of the show thought it would be the most shocking not just to the audience, but to the character of Stefan. If he had revealed himself to be the progenitor of the Salvatore bloodline, making Stefan his doppelganger BEFORE the finale, it wouldn't have made for a great cliffhanger. :) Hope that helps.