When they did the big software transition a few years ago, they deleted as many old threads as possible to keep from having to transfer the whole batch of data.
At that time, the decision was made to preserve certain forums which had a lot of accumulated archival "wealth" so to speak. Prospects, History, Business were three of the priorities. As you have noticed, the Lounge was the lowest priority because most of it is just pure drivel.
The team forums fell in the middle, deleting a lot of older data except for threads which were hand-chosen by the mods for their historical significance or whatever. That's why for example there are 9 threads on the Canes forum from pre-2012, including the Stanley Cup winning thread and a couple of prospect/draft related ones.