It's community content and bandwidth can't be that big? can't someone mirror the backend too?

It's community content however due to UG's monetization they are subject to tablature licensing rules (hence why some artists can't be found on there).

If you were to gain attention hosting their scale of tabs, you would likely run into legal hurdles pretty quickly.

For example a full archive of most sites dating back to the 90s already exists here: https://tabarchive.mikethetech.com/index.php. However he does not have permission to actually share it.

And? You say that like punishment or retribution is expected for breaking someone's ToS.

I hope you don't read this comment because that's breaking my terms of service.