Pedro Pages
Pedro Pages was called up primarily for two reasons. First, his defense throughout the minors has been touted as excellent. His arm is an absolute cannon, and his blocking and receiving are strong for a prospect. His game-calling has also received rave reviews.
Second, Pages serves as the primary backup catcher for the time being. With how hot Ivan Herrera has started off the year and how consistent (up until the last series) Willson Contreras has been, both have needed to be in the lineup to spark the offense. Herrera has cooled down a bit, and key starters are returning soon, so the DH role will become a nomadic spot with players taking at-bats as rest days from the DH position.
Once Matt Carpenter, Dylan Carlson, and Tommy Edman return, three players will have to be squeezed out. In my mind, it will be Jose Fermin, Michael Siani, and either Pedro Pages or Brandon Crawford. John Mozeliak and Oliver Marmol may be hesitant to release Crawford given their financial commitment to him. Pages is still technically a prospect with options; that makes his demotion easier than Crawford's release.
Pages's demotion is by no fault of his own. While he hasn't recorded a hit just yet (he has 1 walk and an RBI), his role on the team isn't to provide decent offense. He is simply a safety valve; he is the third-string catcher, and his role is slightly more important when both Willson Contreras and Ivan Herrera start.
Pages can still develop into a strong major league catcher. His time to be in the majors just isn't right now, especially as the Cardinals' search for offense continues.