Question

what are the reasons why Pluto can not considered as a planet?

Answer 1

because it was too small and was out of its orbit

Answer 2

There are many reasons that would be considered for not considering Pluto is a planet rather than dwarf planet :-

1) It is very small as compared to another planet.
2) It does not follow its orbit and follows others .
3) It is very far from Sun for being a planet in Solar System.

HOPE I WAS HELPFUL TO YOU :-)