They don't have to add them at all. They could easily say "tough luck, you missed the event so you miss the items."

There are very few exclusively offered items on the mogstore. Fantasia and namechange are very reasonably there. I don't really agree with Sleipnir and the other store-only minions (but those are no different than the plushy-given ones or the soundtrack ones) because I think those should be attainable in game at first.

But old holiday things? Let them be on the mogstore because it's a convenience being added for those people who initially missed the event.