Although ceramic can technically be sharper, and technically is harder, it’s both very difficult to get a sharp edge and that edge is extremely brittle and chips easily.
Steel can be made much more precisely easily, and the sharpness has toughness that won’t degrade as quickly. This lets you make an edge that cuts much cleaner, which leads to better shaped particles. Once you start “mashing” the particles instead of cutting them, grind size distribution gets a lot worse and you start getting more “fines” which leads to muddy cup profiles.
Also, because ceramic burrs are used on lower end grinders, the tolerance on the burrs and the other stuff around the burrs (alignment, shaft, etc) is much worse overall which impacts repeatability, and because you are continuously grinding, repeatability (play or shimmy) translates to worse grind size distribution.