It's orbital prejudice, is what it is. Astrologers are butt-hurt that Pluto doesn't have a nice almost-circular orbit, like the other planets, and that its orbit is tipped out of the orbital plane of the other planets. In other respects, Pluto easily qualifies as a planet.
I think they also claimed it is smaller than our moon. Of course, our moon is speculated to be the remnants of a planet that was smaller than earth and collided.
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