The “Planet” Pluto was discovered by scientist in 1930. Even then, there was argument over whether this little ball of ice should be considered a planet. It was decided it should, mainly because since the gravity of the eighth planet, Neptune, was found because it was tugging on the seventh planet, Uranus, and then Pluto was found when its gravity tugged on the orbit of Neptune. Pluto was just a continuation in the time-tested practice of discovering the existence of planets from the behavior of other objects in space.
In August 2006 the International Astronomical Union downgraded the status of Pluto to "dwarf planet." This means that from now on only the rocky worlds of the inner Solar System and the gas giants of the outer system will be “named”as “planets”. The “inner Solar System” is the region of space that is smaller than the radius of Jupiter’s orbit around the sun. It contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth, and Mars. The “gas giants” of course are Jupiter, Saturn, Neptune, and Uranus. So now we have eight planets instead of the nine we used to have.
Pluto is made up mostly of ice and rock, but not enough rock of count as a rocky planet, plus all the rocky planets are near the sun. Pluto is the farthest “planet” from the sun of the current nine known planets. In fact, less than 1/3 of Pluto is rock. It really just is this tiny little ice ball floating out in the depths of space. If there were other planets made mostly of ice too, maybe Pluto would fit in more, but as far as we know it is the only planet made of mainly ice, which just doesn’t fit in with the other planets. So one reason not to consider Pluto a planet is; the different composition of the planet from all the other planets.