It was disqualified as a planet because orbital dominance was not achieved in the case of Pluto. Orbital dominance means that a the planet candidate should have remove all the small bodies from its orbit, by impact, capture of gravitational disturbance.
According to the IAU in its resolution B5 (IAU is the International Astronomical Union; in particular it is in charge of naming celestrial bodies), a planet is a celestial body:
- in orbit around the Sun
- with a sufficient mass so that self-gravity overcome rigid body forces (that assumes a hydrostatic equilibrium)
- has cleared its neighbourhood.
As you see in the resolution B5, this definition is specified for "Planet in the Solar System". It is mainly because it does not really make sense now that it is already challenging to observe planets, so the distinction between a planet and a dwarf planet is not yet crucial for exoplanets. So the official definition of an exoplanet by IAU makes mainly a difference between a planet and a brown dwarf.
However, I guess the IAU definition of a planet should be kept for the other planetary systems if we are able to detect objects such as dwarf planets.