If you speak of what matters most for regiments, its 1v1s, its a long tradition, it will probably never change, just look at the community in general, both in term of popularity and involvment.
Most people that pick gf on this thread say it is boring or so, it is but that is not the question asked.
So first, groupfighting is rising because it is great, fun, more skill based and somewhat
refreshing to most people in comparison to LBs and 1v1s that often end up being boring (camping or skirmish), especially after some time, lb are fights where you have not much ways to influence it as a player (shooting, the leader command your movements,...unfair numbers in melee) which is different in GF and people appreciate that.
It has to be said that groupfighting has its own community, regiments do groupfighting because it's great and fun but for the overwhelming majority (i've seen almost no very GF oriented regiment such as 85e) their major competition are NWL,1v1s, and LB based tournament and leagues.
More importantly, i personally don't consider that regiments are really part of the competitive aspect of the groupfighting and by competitive aspect i mean gf tournaments.
Their competitive tradition leads them to NWL and 1v1s, also the fact that there is very few regimental gf tournaments in comparison to the most that are standard groupfighting (in which regiments are few to take part in nor doing anything significant), the regimental structure doesn't fit well with the gf community (5v5,7v7,8v8..)(non-partisan gf team to be competitive), make me definitely think that regiments are not part of the groupfighting community, the groupfighting teams exist if you like gf and want to focus on it you get to join one, there are group out there totally devoted to GF, the gf teams are the group that are truly concerned by whether or not you are better depending on gf results, you have to be involved in the community of your main type of competition for it to be the factor that say "you are better than them", its only good sense to be valued relatively to your historic and main type of competition
We could compare regiments to a veteran pro boxer that is still boxing (1v1s, LB) that has a long carrer behind him (tradition), he discover mma (groupfighting) he likes it, its refreshing and found it more nervous than usual boxing fights, he start fight in mma (which the popularity is increasing) but not professionally or a very few (tournaments) because its not his primary and traditional sport, he eventually beat an other boxer that started mma in an amical fight , so then do we consider consider him as a better fighter because he beat an other boxer in a sport that is not boxing but still is about fighting ? no, we judge the boxer on his boxing carreer.
We judge a regiment on his 1v1s carreer.
Following this logic, we can say this regiment is better than this one in gf because of gf result, but not a better regiment, as the regiment is originally fit for battles and linebattles and then to its competitive extend 1v1 competition.
Even in term of meta, i know this game enough to tell you that there is real differences that occur real consequences on result between 1v1s and groupfighting.
You're not beating a regiment in 1vs1 because you did in gf, very very far from it.
GF popularity increased a lot, is widely played and thats for the best but 1v1s and LB remain the big deal for regiments.