Because your game is Waaay too old. You have the pre-DX release 5 - the "FINAL" on your release version meant it was the final one before DX arrived. The release after that was DXR1, and there have been new ones all the way up to the current DXR8. At one or two points along the way, they made updates to the updater, and your game doesn't have them, so it no longer communicates with the update servers. Grab yourself a new DXR8, taking care to move over anything you currently have in UserData and mods/MyMods.
thanks for the explanation