I have a feeling it's the former. They similarly waited a few months after X/Y came out to launch the bank, so the update not being simultaneous with Sun/Moon doesn't really surprise me.
But wasn't Bank not even announce our possibly even an idea at X/Y launch? I thought it popped up years later. At this point I expected we could transfer our Pokémon to sun/moon the first day. Is there Bank support for OR/AS? If so, when did it start?