Why is Pluto no longer a planet?
Pluto is still considered a "dwarf" or minor planet (of which there are at least 3 in our solar system), it has just lost status as one of the major planets around the sun. The Astronomical Society voted about 60% to 40% to declare Pluto a minor planet due to several things. It does not have an elliptical orbit around the sun and will pass through the orbits of its neighbors at times. It is also relatively small, but so is Mercury. It does have three small moons, but based on evidence, most astronomers consider Pluto to be a moon that has escaped the gravitational pull of Neptune in the distant past. At times, Pluto passes closer to the Sun than Neptune. The main reasons that Pluto is now considered a "dwarf planet" are its size and the fact that it passes through the orbit of Neptune. This is based on the new definition of planet by the IAU.
I think simply because Pluto is so small
According to National Geographic online, planets should be large enough that they dominate their neighborhood. Because Pluto's moon Charon is half as large as Pluto itself, it cannot truly be said to dominate the neighborhood (consider other planets are much much larger than their moons). Also a planet should be large enough that its own gravity has caused it to become round by drawing in asteroids. Lastly it has been brought up that Pluto's orbit around the sun is not orderly enough to be considered a planet.
So those are some of the reasons why Pluto lost its planet status.