Several reasons:
1. It looks messy, and is irksome.
2. The portal makes up for it.
3. I don't really see any use for it. What would you put there? Everything else, such as recent posts, is on the portal.
4. It's not exactly attractive.
5. It's annoying.
I don't want to further that, but I could probably go on and on. No offence, but I've seen forums like that and it really puts me off. Statistics go on the portal.