Because when Pluto was discovered, they thought it was alone out there. Then they found that Pluto was a lot smaller than first thought (mass 1/5th of our Moon, roughly the size of Australia), and that there were a lot of other objects only slightly smaller in similar orbits (the Kuiper belt). And then we found Eris which is just as big as Pluto (slightly smaller in diameter, but more mass). So either they are all planets, potentially thousands of them in the Kuiper belt, or we redefine 'planet' so Pluto, Eris, and similar bodies are excluded.

So now a 'planet' is a) orbiting the Sun, b) mostly spherical, and c) mostly alone in its orbital neighborhood[1].

Pluto, Eris et. al are still more than mere rocks, they have been reclassified as dwarf planets[2].

It was a similar story with the asteroid belt: They found the first asteroid (Ceres, I think) and called it a planet. Then they found another, and another, and soon it became clear that there were a very large number of small bodies in similar orbits. So they decided that they weren't planets after all, and called them asteroids instead[3].

The difference with Pluto is just that it happened so recently.

[1] https://en.wikipedia.org/wiki/IAU_definition_of_planet

[2] https://en.wikipedia.org/wiki/Dwarf_planet#Dwarf_planets_and...

[3] https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)#Classific...