Question

Why is Pluto considered a dwarf planet?

Answer 1

Because it didn't meet the criteria in being a planet .

Explanation:

Based on IAU a planet is a celestial body that:
1. orbits around the sun. (ellipse orbit)
2. has sufficient mass in order for itself gravity to overcome rigid body forces such that it assumes a hydrostatic equilibrium shape. (round in shape)
3. it has cleared the neighborhood of its orbit. (dominant object in its orbit)

while...a dwarf planet is the same as criteria 1 & 2 and:
3. it has not cleared the neighborhood of its orbit
4. not a satellite of any other planet.

Also, if the pluto is not considered as a dwarf planet then the other planets in the Kuiper belt needed to be a major planet too.

reda this for more infos: http://missionscience.nasa.gov/nasascience/what_is_a_planet.html