Fire Festival is a night of celebration for the Wano folk, the only time they can actually somewhat live like humans throughout the course of the whole year. A night they can let go of all their worries and dream. Having them see the whole struggle with Onigashima above the Fire Capital would've not only ruined the Fire Festival but driven everyone in the capital into a despair, with would've ultimately taken that privilege away from them.
So I think Oda is purposefully tryna keep the Wano citizens ignorant throughout this whole ordeal.
The way tings are setup, the Wano folk are gonna way up tomoz & find that all of their wishes have come through. Somewhat like waking up from a long-ass nightmare that didn't exist in the first place.
Doing tings this way I think would be a lot more impactful/emotional. You think u're gonna wake up from a "dream" (i.e. the fire festival) into "hell" (i.e. daily life on Wano) whereas it's actually gonna be the opposite ^^