Question

why pluto is called dwarf planet?

Answer 1

Pluto become a dwarf planet because its size is comparatively too small and sometimes it violates the rule by getting into the orbit of neptune and its distance from the sun is more and pluto 

and there were many bodies similar to the size of pluto and some of them were bigger than pluto and they had certain different characteristics therefore the scientists decided to consider pluto as dwarf planet 

there would be chances of collision of pluto and neptune but  stable orbital resonance with Neptune prevents them from colliding

Answer 2

because it did not meet the three criteria the IAU uses to define a full-sized planet. Essentially Pluto meets all the criteria except one—it “has not cleared its neighboring region of other objects.”