Because etherpad isn't meant to be persistent.
To archive a pad, fork this repo, clone it locally, then run:
./archive.sh etherpad_url
For example if your pad is at https://public.etherpad-mozilla.org/p/example you would run:
./archive.sh https://public.etherpad-mozilla.org/p/example
Then you can git add everything, git commit and git push to your fork, and finally make a PR against the gh-pages branch back here to get your page added to the archive.
Your page will be available at
rik.smith-unna.com/etherpad-archive/{{original_url}}
For the example above, it would be found at
rik.smith-unna.com/etherpad-archive/public.etherpad-mozilla.org/p/example
It will also be added to the index page at http://rik.smith-unna.com/etherpad-archive