The question you're asking about is protected, which means it cannot be answered by users who have less than 10 reputation on the site. The 100-point association bonus doesn't count, so as of right now you'd need to earn 9 reputation on the Game Development site
Questions are generally protected when they become popular and as a result start to attract low-quality responses (such as commentary posted as answers or spam).
The protection banner is displayed underneath the question:
It's arguable that it should also be displayed in some form by the answer box. It certainly seems like it would be useful for questions with a long answer list. But I don't think other SE sites do that? If I'm wrong, that's unfortunately not something we can fix without SE's intervention.