Ceres has a gravity of 0.028 G, which on default human race settings requires low-gravity infrastructure to colonize. You can see this by the "LG" appended to the colony cost in several windows.
Io has a gravity greater than 0.1 G, which on default human race settings is considered habitable and requires only normal infrastructure to colonize. This is why you can put population on Io, but not on Ceres.