Question

why pluto is not a planet anymore

Answer 1

According to new rules adopted by the International Astronomical Union, a celestial body must meet the following criteria in order to qualify as a planet:

A planet must be round.A planet must orbit the sun.A planet must have “cleared the neighborhood" of its orbit. This means that as a planet travels, its gravity sweeps and clears the space around it of other objects. Some of the objects may crash into the planet, others may become moons.

Pluto follows the first two rules: It is round, and it orbits the sun. It does not, however, follow the third rule. It has not yet cleared the neighborhood of its orbit in space. Because it does not follow this rule, Pluto is no longer considered a planet.

Answer 2

Pluto is not consider as planet anymore because it doesn't follow three criteria of IAU i.e
Three criteria of the IAU for a full-sized planet are:

1) It is in orbit around the Sun.
2) It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
3) It has "cleared the neighborhood" around its orbit.