Misty wasn't the Cerulean Gym's official Gym Leader though, and if you recall, Brock knew her three sisters were the Gym Leaders but he refused to tell Ash about them in that gym battle episode. I think Giovanni might have been the only exception since he was all mysterious about things lol.
And I don't like Iris to be honest, I'm just glad she won't be as bland as I had originally thought she'd be
It could also mean it will take longer for Iris to become an interesting character. Once Ash is done with the last gym he heads directly to the League.