Well, the problem was that we kept finding stuff far out with a similar mass to Pluto (which is much smaller that our Moon) so we’d either add a bunch of stuff to the solar system or take Pluto out. There is always a point from which you have to cut, and Pluto didn’t make it.
I think its a reasonable, even if completely arbitrary, choice. Its simpler to only have 8 planets.
It’s not a planet. If it was a hundred other things would be planets. It’s sad though. It always was to me. It was less than a Plutonian year between it’s discovery and getting down graded as a planet… It never even got to it’s first birthday.