Just a guess, but the site is more than just the forums. The forums are probably a part of the site, and if you're moving from one version of site hosting software to another (which I believe happened), then it would be "all or nothing".
Given TPTB chose to migrate the contents of the previous version of the forums to this version of the site, there would be no real way to run in parallel. If they waited until all was up, that would have kept the forums down all that time as well.
In addition, I'm pretty sure CGL doesn't have teams of highly qualified webmasters on the payroll - I think Herb, Randall, and maybe Herb's cats are the only permanent staff, actually. You'd be amazed how much gets done by volunteers.
W.