A380 was already phased out once, I would imagine any work on Allegris for it would have been stopped at that point. So even if they expect the A380 subfleet to last now, it would first have to catch up to the rest, and I can't see how that would not result in even more delays in already criminally delayed Allegris rollout. Going for an off-the-shelf solution is the right move in this situation.
(frankly I wonder why they didn't go for off-the-shelf on 747UD also. Maybe the cost would be prohibitive with such a small order, given that only one such product exists in the wild, or they think the decrease in capacity is not worth it.)