I think it's because she views Katakuri as someone who can threaten her position, and for him to move in to protect her in front of all the Guests that were attending the Wedding would not be a good look in her eyes.
No other Yonko seems to care about stuff like that. Whitebeard didn't care about Marco and Jozu stepping in. Kaido didn't care about Jack stepping in. With Big Mom and Perospero, she didn't care. But with Katakuri, it was an issue. There is a reason for that, and my bet is it's a personal one.