Why weren't Ceres considered a planet?

Answer 1

Ceres is not considered a planet, since it does not dominate its orbit. There are other sizable objects at that distance from the sun, in orbit around the sun. It is a dwarf planet though.

Answer 2

Ceres was initially considered a planet when it was discovered in 1801, but as more similar objects were found in the same region of space, it was reclassified as a dwarf planet in 2006. This reclassification was due to the realization that Ceres shared many characteristics with other smaller celestial bodies in the asteroid belt, such as irregular shape and lack of dominance in its orbital path.