We don't have a dedicated forum for networking alone because, when we survey threads, we find that there are not enough threads about the topic to justify it. The more forums we have, the more clutter there is on the forum home page, which some people think is already too long.
Some forum topics (like Apple TV and MobileMe) just barely make the cut, and some don't, and we reevaluate it from time to time. This issue has come up repeatedly over the years for specialized topics.
As a compromise for networking, we combined forums that are often but not always related: servers and networking.