He wore suits up until the in the invasion and don't think he has worn one since. Think he has stated he won't wear one until they have won the war. He won't wear military fatigues as he isn't in the military, so wears this outfit to show he is a leader at war.
I know it’s irrelevant and probably a stupid stance.
Zelensky not wearing a suit to this has really pissed me off for some unknown reason.