@VINT64 Because Komi wanted him to.
He bluntly refused when Najimi told him to, but it jump-cuts from Komi looking at him expectantly/excitedly to him wearing the uniform.
I can't think of any other manga off the top of my head where the guy doesn't wear street clothes to the festival.
That's always bothered me, so it's nice to see this.
@Kaarme
I have no problem with that mechanism, or the "exploit" that you mentioned. It allows a group to boost a late-joining friend so that everyone can play on a more equal level. Playing with someone across a large level gap is generally not very fun for either party.