A group of foreign a-holes suddenly wanted the huge controversy (/sarc) about the definition of planet settled, otherwise the entire science of astronomy was going right into the bin (/sarc), and they just happened to concentrate their fire on the only planet that to that point had been discovered by someone in the US. The first proposals were too hostile, so a compromise was reached.
Well, regardless of their political views, their reasoning is very much grounded in actual astrophysics. Planets form by sweeping up all the material in their respective orbits, more or less dominating them. Pluto formed in a manner more similar to asteroids in the asteroid belt. It has far more similarities to them and other KBO’s (Kuiper belt objects), so it’s not a far stretch that it deserves to be classified among them. But we’re still discovering things about it, so who knows...