Why is Pluto not considered a planet?

Pluto was discovered in 1930 and long considered the ninth planet. In 2006, the International Astronomical Union defined a planet as an object that orbits the Sun, is nearly round, and has cleared its neighborhood around its orbit. Pluto meets the first two criteria but not the third, because it shares its region with many icy bodies in the Kuiper Belt. It was therefore reclassified as a dwarf planet. The decision remains well known because Pluto had been part of the planetary lineup for decades before the definition was formalized.