Because they discovered Eris and instead of calling it the 10th planet and then have to name the other small objects they were finding, they decided to change the definition of "planet". To be a planet the object has to "absorb" all of the material around it, but how do we know it's not in process of doing so? It takes a long time to do this.
Astronomers discovered a whole swarm of objects in our solar system that are bigger or about the same size of Pluto. And these objects stretch continually to smaller and smaller sizes. Rather than call all these new objects planets, and dilute the meaning of the word "planet", and force school kids to memorize hundreds of planet names, they decided to create a new class of objects known as "dwarf planets".