Sarah was replaced at the height of her popularity, and has a promising future for high political office. Very deliberate person who people trust.
Kayleigh is the best package to go toe to toe with the media in the current environment. She has incredible experience for her age and an education resume that puts any reporter to shame. Can't be attacked for her looks, either, which is a card the media animals pull at will.
She is clearly intellectually superior vis a vis the reporter class in general. The exceptions, such as Chanel Rion and Emerald Robinson know how to ask questions that boost both Kayleigh's and their own intellectual profile. Same wavelengh.
Press secretaries thus far have been like the pitching sequence in a baseball game. Spicer was the starter, Sarah the long man, Stephanie G the setup man and Kayleigh the closer. All had/have different roles.