Question

why is pluto not a planet

Answer 1

it is too small planet in the solar system so it is considered as dwarf planet

Answer 2

The understanding that Pluto is only one of several large icy bodies in the outer Solar System prompted the International Astronomical Union (IAU) to define formally in 2006 what it means to be a "planet". This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category .