From the pinned GotY FAQ -
6. Will it contain other Downloadable Content, such as Mehrune's Razor, Vile Lair, Thieves Den, and The Wizard's Tower?
No. On a recent episode of the http://www.oxmpodcast.com/, Bethesda Softworks Executive Producer Todd Howard had the following to say about the Oblivion Game of the Year Edition when asked why the rest of the released downloadable content will not be included:
"The two reasons are, these are the ones everybody wanted [Knights of the Nine and Shivering Isles]. Secondly, these are the two that are designed to be in the game when you...if you were to start a new game, see what I mean? So if you were to have, if you download all the downloads we have and you start a new game, it starts throwing up all of these journals, it acts kinda weird....do you know what I mean? It's like you have this, you have this, you have this, you have this, so we have to go back and kind of massage them a little bit, to start differently. So we view Game of the Year as something [for] someone who's, for the most part, not bought Oblivion before, and now there's a better version of it they can buy. So they go out and buy it, and we want that first-time experience to be smooth."
He's rambling a bit, but you get the idea.