First question, how do you know that it is clearly a planet? What makes you so sure?

There are many celestial objects orbiting the Sun. Not all of them can be classified as a planet. One of the main definitions for a planet is that it needs to clear everything in its path. It has been concluded that Pluto is not large enough to be able to do that.

https://www.loc.gov/rr/scitech/mysteries/pluto.html

The International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet 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.”

So, the 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.

Pluto meets only two of these criteria, losing out on the third. 

Because the size of pluto is so small, they think that it could possibly be a dwarf planet. I have to agree with you, it's just a bunch of c*** that they name it a planet first, and then take that away. Can't imagine how it must feel. Poor Pluto.