It doesn't actually. That's a proxy, not a mirror; designed to make TPB available from countries where it's blocked. Try a search, it won't work.

https://proxybay.info/

However, there's a script where you could download the whole TPB, it's only 90 Mb. That's possible because now they're using only magnet links, so a torrent is pretty much just a URL.

http://torrentfreak.com/download-a-copy-of-the-pirate-bay-it...

So if someone have a recent copy, it would be easy to setup a mirror.

Or people could just use one of the dozen torrent site still up and let the whack-a-mole game continue.