Honestly, I think it's because the quiz is 'more convenient' to fill out than the comment box. Think about it. If you want to post a comment, you need to think about what you'll say, how you'll say it, maybe proof read it to make sure you didn't mess up, etc.

That's not convenient for a lot of people.

The quiz on the other hand is just a multiple choice thing. Click a few radio buttons, press submit, done. It's like with site polls; they're quick and easy to fill out, so you get more people voting than you do commenting.

May also be the novelty factor too. People may just want to try out this strange system they've not seen before, whereas as comments are just old hat.

Just some thoughts here.