Because the way you have it set up now makes any meaningful discussion impossible. Just a straight line scrolling down, from newest to oldest. And I have a lot of trouble figuring out why this design would be used at all. Only reason I can think of is it being intentional, with the goal of making it hard to find things anyone has posted about before.
But that would be a pretty awful thing to do to fans, so I figure it must just be an oversight. And you guys really should consider a forum design that actually promoted community discussion, rather than hindering it.