scottro had it mostly right.
The Forums started out not only because other forums failed or went away, but also because people at FreeBSD felt it needed its own community support beyond the mailing lists.
This was at a time when mailing lists fell out of general use, and forums became more prevalent (this was also when things like blogs and wikis became all the rage).
So FreeBSD developer Brad Davis rented a VPS and installed forum software (with the admin user joining as member #1 on Sep 26, 2007), all on his own dime, but of course under the freebsd.org domain.
This was before the Foundation supplied things like central funding for FreeBSD (other than maintaining FreeBSD itself).
A couple of developers and volunteers were quickly added on as administrators and moderators. Little has changed over the years, activity levels may have shifted.
Because the Forums were installed and run separately and developed their own little ecosystem over the years (maintenance, installation, monitoring, external access, scripts, firewalling, etc. etc) it is almost impossible to integrate them into the current FreeBSD ecosystems and clusters. Many exceptions would have to be made, and this works perfectly fine.
The Foundation has taken over the financial side, so the software, plugins, licenses, hosting costs, are all taken care of.
Even though the Forums have formulated their own set of rules and regulations, and are run very much independently from the other FreeBSD infrastructure, they are very much "super-governed" by the FreeBSD organization ("Core"), though there has never really been a situation in which "Core" had to intervene or put the hammer down, or anything like that.
It's best to consider the Forums a fully-owned subsidiary or a satellite in fixed orbit around FreeBSD-proper.
And yes: 
it's official (r/FreeBSD is not). It is linked directly from the navigation bar at freebsd.org, and from the Community page at the FreeBSD Foundation.
The reason why there are not a lot of developers here is two-fold:
1) they 
really like their own historic channels (mailing lists, IRC channels, and development platforms)
2) the Forums are specifically 
not about development, they are here for user/administrator support and community-building; some developers do interact at that level, but it's not their daily FreeBSD driver.