The short answer is that Pluto isn't a planet for the same reason that the Sun and our moon aren't planets and the Earth is, even though the sun and our moon used to be thought of as planets while the Earth wasn't. It's also the same reason as to why Ceres and Pallas aren't planets, either (even though these two used to be planets in our solar system, even if you've never happened to hear of them). (If you want "someone" to blame, then blame Eris, which - somewhat appropriately is named after the Greek Goddess of Discord.)
Therefore, the question above technically isn't the right one to ask. A more correct question is, "What is a planet?"
CPG Grey explains why definition matters in explaining why Pluto isn't a planet anymore (or, what I believe is the root question, what the heck the work "planet" means).
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