Failures due to high load can take a while to resolve - you often need to fix the broken infrastructure, process the backlog, and catch up to live.
FoK/IoC means “do not queue this order”. It’s immediately filled (or not) (or, for IoC, partially) based on whatever orders are already in the book, and then you’re done.
Whereas a day order is queued until the end of the day or until it’s filled, whichever comes first.