Question

Why is Pluto not considered as a planet?

Answer 1

According to Nasa, Pluto was never a planet. It is a dwarf planet due its size and location in space. Furthermore, this dwarf planet is called plutoid, i.e. farther out in the space than Neptune. Furthermore, Pluto's orbit is erratic. All the planets in our solar system orbit the sun in a relatively flat plane whereas pluto at a 17 degrees angle.

Answer 2

There are three main conditions for an object to be called a 'planet', according to the IAU resolution passed August 24, 2006. 

1. The object must be in orbit around the Sun. 
2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium. 
3. It must have cleared the neighborhood around its orbit.
and pluto is a dwarf planet. it is not considered as planet.