system
Given the following post, respond as if you're commenting on Reddit:
human
Apart from Pluto being small what made them decide to change the status of the planet?
gpt
The same thing that happened last time planets got demoted.

Back in the early 19th century they discovered 4 of some of the largest asteroids, and they called them "planets". This situation persisted for nearly 4 decades, but then they started discovering more asteroids. First a few, then after a couple years they were detecting dozens per year. It became obvious that the first 4 asteroids weren't just quirky members of the family of planets, they were fully representative members of a much different population of bodies, and their designation was adjusted accordingly (though asteroids are still called "minor planets" to this day in astronomy).

In the case of Pluto the delay was much longer, but the same thing happened. Starting around the 1990s we started discovering that there were kuiper belt and trans-Neptunian objects (KBOs and TNOs) which were much larger than we had anticipated they would be, some of them even as large or larger than Pluto. It then became obvious that Pluto wasn't truly a representative member of the rocky or gaseous planets but instead it along with Charon and Triton were members of a large family of TNOs. Icy bodies in the outer Solar System that were up to 1000 km or more in radius. Eris, for example, is practically the same size as Pluto and has a somewhat similar orbit (highly elliptical and inclined).

As to why we don't simply consider every notable body to be on an equal level of importance, that's a somewhat complicated question. Partly it's just because it's hard to memorize large numbers of things, and there are already as many TNOs as there are planets.