They weren't expecting to get interrupted by an invisible God of Speed, so weren't in a rush. Plus, a mask that size could've been booby-trapped, I wouldn't want to pull that off without having a bomb-sniffing dog check him first.
Heck, forget unmasking. In the 33 seconds between Alchemy being disarmed and the ground starting to shake, why didn't any of the cops present actually say something like "You're under arrest" or restrain/cuff him or start to read him his Miranda rights? I'd consider that to be more unrealistic than not immediately trying to undress him.