I think they're just mad that the Sumerians said there were 10 planets in the Solar System and they found Chirion a while ago. Trying to save face now? Small = not planet? For heavens sake, all non-kidding aside, as a very wise Libra friend of mine once said.
Here's the thing with Pluto as I see it, it has an erratic orbit and doesn't 'behave' as other planets do (kind of like yours truly, I blame it on my Aquarius ascendant). Doesn't revolve around the Sun... but rather something called the Asteroid Belt -- that's the Galactic bellybutton you know. So, it orbits and it does it in our solar system, (that would be obvious).
That's where we get the idea in Astrology that Pluto leads us to different levels of awareness, deeper depths, the truth that lurks behind what is apparant. Astrologers see it as something to help change perspective, (something astronomers could use more of perhaps?
So, not a planet because it's not normal. Glad they don't tell us we're not people if we're not normal. No one would be. What's normal. It's agreed by those who put thought into it, that it's a relative concept for humans --- we need a commission on Planet Rights! It's relative for them too!