It is popular right now because it's new and people want specific rewards, but eventually most things die unless they're on the roulette page.

Some people only look there after they have the rewards they want, especially if they have limited time to play and only want to run the stuff that gives a nice bonus each day.

When Rival Wings returns in 6.2 it should have a daily bonus too, so then all 3 types of PvP are represented on the roulette page instead of only frontlines.

Bonus suggestion: keep obsolete pvp modes around as custom match only, so preformed groups can still do them just for fun even if they aren't part of the current progression. Seems a waste to throw away those feast maps like lichenweed, feasting grounds and that crystal tower one.