1) Frontlines is splitting up people because they have to choose between losing all wolves den progress to play with friends or playing with randoms.
2) The rewards from doing other activities far outweigh the rewards from doing frontlines
3) The pvp gear does not work in frontlines, so pvp gear is vanity only for the most part.
4) This is Final Fantasy, there is already a low want for pvp.
5) Sorta like 1, but having to find 20+ people in the same grand company takes forever to que up. people getting 30+min ques on it.

Those are the 5 reasons I feel it is not done as much.