Question

Why is pluto called a dwarf planet ?

Answer 1

It is first of all, not called a "planet" anymore, as it was too small to be considered. It WAS called a Dwarf Planet as it was the smallest among all the other planets.

Answer 2

Pluto is called as a dwarf planet because of the following reasons:
1.It has neglegable mass as many other small planet have,which are out of  our solar system
2.It does not rotate on its own axis,some times moves into neptunes orbit called kuiper belt.