I don't know, but I think this may give a hint!
When I got my beta code it was like this

Final fantasy xiv1: then code was here
Final fantasy xiv2: nothing
Final fantasy xiv3: nothing
......
Final fantasy xiv8: nothing
I guess the beta codes that was sent ment to be sent to final fantasy xiv 1 accounts and if your ffxiv1 is a canceled account then your code won't work unless you active it. Maybe SE intends to send codes for accounts 2,3..8 in later updates, but who knows it is just a hint!