Because I don't have the money, and it's doubtful I will since my paycheck needs to go to more important things, like food, and buying for Christmas.
Because I'm not really that interested. Oblivion bored me to tears, and part of that was the setting. It doesn't do much for me, so I doubt Skyrim will be an improvement over that.
And because I have a limited amount of time with all the stuff I have going on and, if I'm honest, I would much rather finish the most recent Layton game with my small amount of spare time than I would playing Skyrim.