Why Isn't Pluto a Planet Anymore?

In 2006, the International Astronomical Union voted on its first-ever definition of a “planet.” The definition excluded Pluto from full planet status, downgrading it from being the 9th planet from the sun to a dwarf planet. The move caused an uproar among the public. Why did that happen? What caused Pluto to lose its planet status?

[Gman]

The only part of Pluto’s demotion I don’t like is that people started calling the theorized but as of yet undiscovered planet “Planet 9” when “Planet X” sounds cooler and still works in the unknown, “solve for x” sense, Roman numerals be damned.

[lorensims]

The same reason Ceres is no longer a planet.

[erictavares]

Trust the science! (They got it wrong most of the time)

[charlesrhodes]

I feel like having enough mass to become round from your own gravity makes you a planet. This is supported by the fact that they had to create the term planetoid to describe planets that aren't planets because reasons. It all seems very arbitrary.