Most news sites don't keep articles indefinitely. Some move them to online archives that you have to pay a fee to get into, while others simply don't keep up articles online past a certain date.
Yeah, I figure that's the explanation - but it does seem odd to me that when editors and reporters preen themselves on writing "the first draft of history" they wouldn't take care to see that their work is easily and reliably available to researchers on the web (sometimes it's a hopelessly bad draft, but these articles seem to be important to the real historical record).