Because the forum software has trouble with threads over 2000 posts, so ongoing threads get split into new parts around that point.
vBulletin has a problem with any kind of large database, too. It is very annoying.
I think they've pretty much fired all of Jelsoft, too, so getting things fixed with them is nigh on impossible. First you have to get them to understand what the devil you're talking about, then you have to threaten to show up at their headquarters before anything happens.
It is very annoying, indeed.