Hi deathy,
that's the reason why this new definition became necessary: More and more flying chunks of ice and rock were found in similar orbits as Pluto, some of them the same size or perhaps even bigger, and many more must be waiting out there for their discovery. How could we keep calling Pluto a planet and not recognize those others as planets 10, 11, etc. pp. ad nauseam?
Cheers!
Markus