Explaining why an upgrade didn't work doesn't compromise security. I'd guess moving the data didn't work or they corrupted a database and don't know how to repair it.. Explaining that you don't have proper data backups in place can be embarrassing but with a post mortem you can at least get some trust back (like Gitlab's incident).
Not explaining why you're offline for 24h doesn't help people to trust you
If they corrupted a live database and are not able to recover it they are in a world of hurt. While it is bad form, many people keep their coins on the exchanges and even if the bulk of an individuals coins are offline, they still likely have at least a small amount on their for trading.
If a table that connects user accounts to kraken owned wallets is corrupted and not recoverable people will be out millions. For some that would be the equivalent of your 401k issuing a post mortem for losing all of your retirement.
If this worst case scenario happened they are likely in severe damage control.
Most likely explanation though is that things are just taking longer than expected to upgrade what is by all measures likely a very technical and convoluted system.
Not explaining why you're offline for 24h doesn't help people to trust you