BuddyPress.org

When viewing an actual topic – you’re within the scope of the BP_Forums_Template_Topic class instead. So you don’t have a handle to what is the previous or next topic is (since we are dealing with posts)

If you think about it more – it would be hard to build that reference in a forum environment. What if someone replied to a different topic and it was bumped above the current topic being read? Where as in blog posts – they are static based on the time.