Unfortunately, the long-standing trend is that people run the latest/newest Frontline mode while the others become somewhat obsolete. In this case, Shatter. With Stormblood, SE updated Secure and Seal Rock so they wouldn't be left behind at 50 and 60 (Slaughter was just removed entirely, sadly), but because Shatter is/was where many first got a taste of Frontlines when they started during the GARO event of 3.5, it's all they know, and all they queue for.

Seal Rock (Seize) was arguably the best Frontlines mode, and Secure, though a bit dated is still pretty fun. Shatter actually plays like a combination of the two, but just isn't as good.