I imagine there may have been a time where the roll back to the old system had not yet been completed, so you couldn't use that, nor the new one.
While not an excuse, these backend systems are VERY old.
Just for a laugh: when "new NAIPS" comes online, you will be able to submit flight plans many days in advance. Although I can't remember the number, ICAO set a minimum and many ANSPs around the world have decided to only support the minimum because of storage concerns. (NOT AsA, they are allowing more!)
Yes, good chance some of them are running these systems on ancient mainframes or mini computers with storage measured in megabytes.
If it ain't broken, don't upgrade it it, I guess.
