The answer is pretty simple. As Indigo_Dingo said, Christmas games have a limited timeframe appeal. No sane developer will spend thousands of cold cash to create a game, that interests costumers only for a month. That is simply PC suicide. Just think about it, right after Christmas Eve, when you already unwrapped your presents, eaten the dinner and cookies, the relatives already left, and you are thinking about New Year's, you won't play X-mas games. It's like sex. When the calendar enters December, you get exited, start to prepare for the big day, and get more and more excited, work even harder on the preparations... until you reach Christmas, and after that, you suddenly lose interest.
Game developers are people too, and know exactly why a Christmas-only themed game won't sell. This is why. Of course they make holiday mods to existing games, but that's all about it. Maybe some indie dev will actually go the extra mile to hammer together some cute X-mas game, but don't expect an AAA title, because it won't come. Ever. It's simple business, they don't want to sell at a loss, which is quite understandable.
I, myself, am still happy with the holiday themed mods, levels and episodes, because they entertain me until the Day passes, and then I can return to my StarCraft and CS 1.6 to kill time.
