In 2006, 2% of the world's astronomers, led by a guy with a television show, decided Pluto should not be a planet. Recently discovered Eris was bigger, they said, so rather than make Eris a planet they made Pluto a rock.
They did so by specifically creating a new definition of planet designed to exclude Pluto - "A body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies."
Since Eris was in the neighborhood, that meant Pluto was not a planet. Now it turns out Pluto is bigger, after all, but the logical fallacy of the definition of planet has not changed. After all, are there any planets if "cleared the neighborhood" is a requirement? Nope.
And the definition is too relative to be science. Space.com quotes Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder who notes the definition also sets different standards for planets at different distances from the sun. "The farther away a planet is from the sun, the bigger it needs to be in order to clear its zone. If Earth circled the sun in Uranus' orbit, it wouldn't be able to clean out its neighborhood and would thus not qualify as a planet, Stern said."
So why eliminate one instead of adding two? Unscientific, that they just didn't want to add more, and strangely semantic.
Neil deGrasse Tyson, director of New York City's Hayden Planetarium and one of the first to push for Pluto's demotion, told Space.com, "The word 'planet' has far outlived its usefulness. It doesn't celebrate the scientific richness of the solar system."
Tyson was inspired to go into space science by visiting Hayden Planetarium as a child. Had he been greeted with meaningless jargon about planets being determined based on 'scientific richness' he might have gone into biology instead.
