Because some people just don't want to be bashed on so they keep it short.
Actually this is true. Not one part of the Debates forum is in proper debate form or follows the rules of proper debate etiquette, so why apply a rule? Besides, you can't pin a rule on just the people that reply without pinning a rule on those that make threads.
In other words, if you're going to make a rule where people that participate in debates on the debate forum have to elaborate in their answers, then you have to create a rule where the OP has to make the debate topic elaborate enough to where it makes you answer more than just "Yes" or "No". I've
barely
seen debate topics on that forum that pose a question and then say, "Do you agree or disagree? Why or Why not?" A lot of them only leave enough room for just 1 worded answers.
Besides, people don't and can't seem to respect each others opinions on here, so a lot of people don't feel so inclined to elaborate on their opinion because there's always someone to jump all over them for it. And if they don't elaborate this comes up. It's a lose/lose situation. I agree with Famous, that forum shouldn't even exist because there isn't enough mutual respect going around where it can flourish without everything turning in to an argument.