Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit. Any object that doesn't meet this 3rd criteria is considered a dwarf planet.
Because technically it's not really a planet.Astronomers reclassified it as a dwarf planet because Pluto has not "cleared its neighborhood"(clear its surroundings) like a normal planet would do.
