Why was Pluto downgraded to a dwarf planet?
Pluto was downgraded from planet to dwarf planet because the definition of a planet was changed.
When Pluto was first discovered it was given the status of being the ninth planet.
Since then, other bodies have been discovered in the asteroid belt and beyond Neptune which arguably should be planets if Pluto is a planet.
There are three conditions which must be met to be a planet.
First is must be roughly spherical in shape. Most planets are shaped like a squashed sphere with an equatorial bulge.
Second it must be in orbit around the Sun.
If a body meets these two conditions it is a planet of a dwarf planet. Pluto and other bodies are dwarf planets using theses rules.
Thirdly the body must have cleared its orbit of other bodies. Planets are formed from a disc of protoplanetary bodies orbiting the Sun. These bodies collide and clump together to form a planet. For a body to be a true planet all of theses bodies must have either become par t of the planet or formed a moon of the planet.
Pluto doesn't meet the third condition as it hasn't cleared its orbit of other bodies. Hence the decision was made to downgrade Pluto to a dwarf planet.
Some people, myself included, still regard Pluto as the ninth planet from its historical context. In fact the NASA DE430 data which can be used to calculate the positions of the Sun, Moon and planets still includes data for Pluto.