Whenever a ubuntu version reaches its "end of life", its repository files are removed from the main server and its mirrors.
I don't know why, and unfortunately wikipedia wouldn't let me keep this information there, but there is a place for those old repositories:
http://old-releases.ubuntu.com/ubuntu/dists/
A more general, human friendly page, which also points you to the distro images, is:
http://old-releases.ubuntu.com/releases/
Certainly, you should have a good reason to stay with these versions - and if you plan to do so longer, consider making a local mirror.
Resurrecting end-of-life ubuntu versions
Resurrecting end-of-life ubuntu versions
Jumping out of an airplane is not a basic instinct. Neither is breathing underwater. But put the two together and you're traveling through space!