Why is Ceres considered a planet and not an asteroid?
The definition of a planet was changed which promoted Ceres to dwarf planet status.
When Pluto was discovered it was designated a planet. Since Pluto's discovery other bodies, including Ceres, have been discovered which were considered to have similar status to Pluto.
The definition of planet was changed to include another category called dwarf planet. To be a dwarf planet two conditions need to be met.
The body must orbit the Sun and the body must be roughly spherical.
Most asteroids fail the condition of being roughly spherical.
A number of bodies, including Ceres, meet the dwarf planet criteria and hence were promoted.
To be a full planet a third condition must be met that it has cleared its orbit of all other objects. Pluto doesn't meet this criterion and so was demoted to dwarf planet status.