I saw a dev once flat out reject the idea of ever bringing back any event girls... as a matter of that specific dev's personal opinion on the matter. That same dev, in the same post, admitted that others on the dev team disagreed.

And this was said months before they actually did temporarily bring things back with the game's anniversary. All event girls were back in the epic pachinko all at the same time for a few days at that time. (Not a great looking option from the player's perspective. If there was any one specific one that you really wanted they expect you to gamble on getting her as your guaranteed drop over allllllllll the other possible drops. But they did it.)