Because you lose all the work done since the issue happened. It's very rarely acceptable to just give up and do a full backup restore unless literally everything is gone. If there was just some bug that cause partial issues, you have to find some other way to fix it.

This whole debacle is Exhibit A.

They’re lucky they have a sound backup strategy in place, and that the amount of data lost is appearing to be minimal.

Yea, and this whole thread is very informative for a non-db specialist like me!

Could be that they are restoring from backups - just that that restoring from those backups is very very slow. Atlassian would not be the first where resourcing and testing a speedy disaster recovery strategy wasn't given the highest engineering priority.