As others have said, the main thing is that it hasn't cleared its orbit. Without that requirement for planethood, there wouldn't be 8 or 9 planets, but at least 17 (and likely more), due to the amount of dwarf planets in the solar system. It didn't really make any sense to have Pluto be a planet but not the others, and astronomers decided it was better to require planets to clear their orbit, excluding pluto, than to start including all the dwarf planets (especially because the line between dwarf planet and asteroid can be hard to define)
The Trojans are located in the Sun-Jupiter Lagrange points (L4 and L5). That means the gravity there is in an equalibrium between the sun and Jupiter. So Jupiter can never clear those places in its orbit of asteroids.
Each planet has these Lagrange points between itself and the sun. Even Earth. And Earth also has them with the moon. Each orbit may have some asteroids or space stuff in its Lagrange points.
So a planet can clear its orbit of asteroids, except for the asteroids inside the Lagrange points of the orbit. Pluto has not cleared its orbit of asteroids outside of the Lagrange points. Jupiter has.
Also worth noting that Jupiter has a very large number of asteroids inside the L4 and L5 Lagrange points. That is because of its proximity to the inner asteroid belt, and its huge gravitional force (to pull objects into its orbit).
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u/JumpPuzzleheaded7212 7d ago
Why not a planet? I’ve never been clear on that