When NW and DL merged, the a new fora was created once the frequent flyer plans had merged, with the prior airline fora preserved as sub-fora. Same thing when UA and CO merged.
Since Flyertalk is primarily about miles and points, discussion of frequent flyer plans is paramount and thus, until the frequent flyer plans are combined into (presumably) Advantage (no need for the extra A anymore) later on, no need to combine them just yet.