I'm not sure I understand your question.
When you edit your page, near the bottom, there's an area called "community" where you can select whether you want a "comments" system or a "discussion board." You can change it whenever you want, but I'm not sure if the comments/posts are kept or lost.
The "comments" system is simpler, but more straightforward; I personally prefer it.
The "discussion board" is more powerful; it's similar to a mini-forum, so it's ideal for organizing different topics, for example, a guide thread, an updates thread, etc. But it's less straightforward for reading your users' interactions.
Why not both? I can't say, but I imagine it's a technical limitation. It might not be that simple to implement both systems on the same page. But it's best to wait until someone more knowledgeable about the topic can answer.