One of the best edutainment YouTubers made a video about Pluto's fate, and it made a lot of sense. The story goes that the IAU redefined "planet" to satisfy schoolteachers and keep curricula sane. That was the
primary goal.
So in classical times, the Sun and Moon were classical planets, on the same plane as Mercury, Venus and Jupiter and all the rest. Since the Sun and Moon were planets, they could figure in astrological reckoning in certain ways. And face it, from our observer's perspective, they have a lot in common with actual planets.
In fact, the seven days of the week derive from seven planets: SUNday, MOONday, Mars, Mercury, Jupiter, Venus, and Saturn. (It helps if you list them in Spanish rather than Germanic tongues.) No, there was no EARTHday or URANUSday (thankful for small favors.)
And in school growing up, many of us learned the nine scientific planets as traditionally defined. Uranus, Neptune, Pluto: all great modern discoveries to add to our pantheon. So nine is a good number. Lots of people can memorize nine and keep a set of nine in our heads.
But then they discovered a tenth and an eleventh and a twelfth, and scientists began to get nervous, because something was up: there were, potentially, dozens or hundreds of "planets" to be discovered at this point, among the asteroids and among the Kuiper Belt. That's all well and good: we can name a lot of bodies out there, but school children cannot learn 3 dozen planets in a row! School teachers would never stand for it! So tedious!
So there you go: the canonical list of planets needed a hard-cap and so, they lined up the 8 we had, and found all the commonalities, crystallized those into the definition, and then circumscribed the list so that it would never again grow without bound. It's not unfair to have a lot of planetoids, or dwarf planets, or asteroids and other things, just floating around. But, the good news was that third graders wouldn't need to memorize them all, and third grade teachers wouldn't need to model a solar system of 30 wonky planetary bodies.
And it's that simple, really. 8 standard planets makes for good basic astronomy, good solid astrology going forward, and peaceful middle-school curriculum. Everything else is just semantics.