the answer is simply lack of people.

There are 5 main reasons why PvP is not more populated.

1) Grand Company restriction (probably one of the biggest reasons.)
2) Lack of relevant rewards (unless you love pvp you will not do it, gear is pvp only and token rewards are subpar compared to other venues.)
3) It is a Final Fantasy Game. Comes with the turf this is just not a major pvp game the majority play FF games and mmos for the PvE aspect.
4) Front-lines requires 3 partys... and you are forced to pug, 8 person premade max (never a good idea to force pugs)
5) You end up getting put into premades that are just garbage in terms of what jobs are auto put together.

So you are running into this problem. The majority do not want to do pvp maby 2/5 max want to actually do it in this game, going off asking ppl in my FC and various linkshells. So the base is already smaller. That base is then force segregated based on GC alliance, which is seperating friends and guilds from being able to participate together. And last what is the benefit of doing PvP outside of just having fun?

I am not a huge fan of PvP in mmo, that is what mobas are for in my opinion. But This game lacks a reason to do PvP, especially for those wanting to play with groups of friends and not randoms all the time.