A big problem with XIV's PvP is there aren't enough desirable rewards- or even if there are, you can queue Frontlines non-stop all day or a few times daily and you'll likely have near everything you want from it by the weekend (Wolves Marks). They also have seemingly slowed-down adding new armor/extras to the vendors to twice an entire expansion cycle, making Wolves Marks largely worthless aside from buying dark matter or squadron rationing manuals. Probably doesn't help that Wolves Marks are handed out like candy as participation rewards just for queueing Frontlines with a pulse (for now). Therefore everything Wolves Marks-related has incredibly brief retention.

The achievement rewards from wins on their own sort of help retain players (and aren't simply handed-out, so it encourages you to actually try to win), but once you obtain those rewards I think many players stop queueing out of exasperation or at least take a long break from Frontlines. You can only herd cats for so long before you need to step away from it.

Feast has a bunch of accessibility issues, but I find Feast's biggest problem is the timing of their active ranking seasons. It's already a given that you have a brief window of activity for a few weeks once the season starts to reasonably queue and climb to a particular rank to keep seeing any activity; but what exacerbates the problem is when they release new seasons alongside other anticipated PvE content patches. As a result, by the time many players might decide to start participating in the Feast season after playing the new PvE content (and let's not kid ourselves, the vast majority of players are here for PvE content, not PvP), their queues are already usually ridiculously long to get a match (if you're not at gold, apparently) and it turns people off and away. When the devs release a new Feast season alongside a big PvE content patch, attention gets taken away from it and perpetuates its niche status. To be fair, this has always seemed like a problem with XIV's content cycle where several different newly released content compete with each other for participation (do you grind out the new Frontlines mode while it has insta-queue participation or do you level BLU to the new cap and find parties quickly for the new spells before they disappear from the PF two weeks later?). But alas, that's for another topic. Starting/ending Feast seasons between patches during lulls could probably go a long way to seeing better participation.

And as a side note, they haven't done or added anything new with Wolf Collars in the however-many seasons since it was implemented (except give GNB and DNC weapons, surprisingly), so there's also even less motivation for players to participate if they can't rank.