Why can't I download fedora 13?

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You can download OLD and Unsupported versions of Fedora from the Archive. Of course it's up to you, as the OLD releases are not supported anymore. They do not receive an bug fixes, new features, neither users have support of any kind.

On Fedora Old releases you can find versions back to 7 (so 13 you want, included).

Why do you want to download a release that reached its End Of Life several years ago? Right now, the only two versions receiving any support or updates (even for security issues) are 19 and 20, and 19 will reach EOL about a month after Fedora 21 is released. I don't know for sure, but I'd guess that versions that old aren't available any more because there's almost nobody who still wants or needs them and disk space isn't unlimited. You might go to the unofficial Fedora Forum at http://fedoraforum.org/ , open an account and ask in the forum devoted to EOL versions if anybody knows where to find Fedora 13. I wish you luck, but you'd be better off with a more recent version unless you have a very special need for that specific version.