I'm a huge moon fan, and I acknowledge that it's not a planet. I was a bit upset, but you've got to admit, that astronomers and astrophysicists argue about this stuff is not only hilarious it's fascinating.
Part of the reason it's not is because, as it was mentioned, Pluto isn't big enough to clear it's orbit of debri and the like. Another reason is in 2003 Eris was discovered. Eris is a dwarf planet located in the Kuiper Belt where Pluto is located. It is bigger than Pluto. Had Pluto been kept as a planet, Eris probably would have been named the tenth planet even though it's orbit is even more jacked up than Pluto's.
I feel it's important to note that Ceres was upgraded from asteroid to Dwarf planet as well, thus making Cera Cera the best out of the Amazoness Quartet.
If you want to argue that Pluto is still a planet, you better argue that the Apatosaurus is really a Brontosaurus, the Pterodactyl isn't a Pterosaur, and that the Triceratops isn't just a juvenile Torosaurus. Better work on that last one quick since it's likely but not official.