Blather, rinse, repeat

It has a moon a third of its own size - and is suspected to be a part of it - is of a composition believed to be different from the rest of the rocky bodies within the solar system, has an irregular orbit that more closely matches a chunk of captured space debris than a planet formed with the rest of the system.

Plus it's far too small; Uranus was discovered because gravitational tugs were noticed in Saturn's orbit. Neptune was discovered because those same wobbles were detected in Uranus' orbit. People were looking for the ninth planet because Neptune is being influenced by another large celestial body, one so far unseen but massive enough to influence the orbit of a gas giant.

Its status as a planet was always tenuous at the very most, even going back over a decade. Proper astronomical research has always dismissive and/or suspicious of Pluto's "planet" label.