The main reason why the site is inactive is probably due to the fact that you aren't verbalizing your opinions and you don't essentially need to think on your feet- you'll have the Internet right at your fingertips. However, like Night_Angel stated, persuasive skills are a must and this site could be useful to those who are shy when it comes to public speaking. It also presents a great place to research topics if you're watching two people debate who site their sources. Regardless, the reason could be as simple as the man in the photo at the top of the screen turning on a television set. People enjoy being entertained, and unfortunately debating isn't typically considered prime entertainment.
I think school's should be jumping at this site, and using it constantly. It's a great way for schools to practice debate skills and persuasive skills for English classes, and see the opinions of others throughout the world. It's great for just general practice if you're on a debate team, or can just be fun if you like to argue a lot, or it can help change peoples minds on different subjects. For example, if you post a topic like "Should school start later" and majority of people think "yes" and have a good reason, showing that to school boards could make them start school later.