Question

why pluto is a dwarf planet?

Answer 1

Because it doesn't meet the three requirements defined by the IAU to Pluto be a planet. The third rule says: "It needs to have cleared the neighborhood” of its orbit". Therefore, any object that doesn't meet any of these rules, are considered a Dwarf Planet.

Answer 2

Plus it is tiny (relatively) smaller than the moons of other planets, closer in size to some of the larger asteroids in our solar system, of which there are thousands.

Answer 3

rmariuzzo is not quite right, it meets all but one of the three requirements to be a planet. 1. does it orbit around the Sun (yes), 2. has sufficient mass to have a relatively round shape (yes), 3. has "cleared the neighborhood" around it's orbit (no). It didn't pass #3 because other objects in the Kuiper Belt (where Pluto resides) are also in the same area as Pluto. For #3 to be correct, the Belt would need to be further away from Pluto with no objects entering Pluto's path.