Why does Pluto not count as a planet anymore?

Pluto was discovered in 1930 and long treated as the ninth planet, but later discoveries showed it is one of many icy bodies in the Kuiper Belt. In 2006, the International Astronomical Union defined a planet as something that orbits the Sun, is round, and has cleared its orbital neighborhood. Pluto meets the first two criteria but not the third. It was therefore reclassified as a dwarf planet, a category that also includes objects such as Eris and Ceres.