My best guess is that Payton would prefer to not throw Nix to the wolves. He's a little on the smaller side and his arm could use some work. The problem is he's got Stidham and Wilson as his veterans. Neither of those guys has put forward an NFL resume such that Payton can auto-name either of them the placeholding QB1.
I think it's gonna be an all out battle for QB1 for the 2024 season. The writing is on the wall that they view Nix as their future, so we're only talking this season for both Stidham and Wilson. Zach has the strongest arm. Zach has, by far, taken the most NFL snaps. If he can't earn that QB1 spot for this season, then he's not an NFL QB.