Let me correct a few things:

All Frontlines maps have always been capable of 24 or 72 player options. There was once even a 48, but that was removed quite a while ago.
Secure (and the Deathmatch style Slaughter) sat dead at lv 50 for all of Heavensward, and Seal Rock - while arguably the best mode - was later outdated with Shatter. In order to ensure Secure (Slaughter was just removed, sadly) didn't stay dead at 50, and neither Seal Rock or Shatter would die at 60, they simply lifted the level restrictions of both.
I've said it and proved it many times before, but "People only queue for Shatter because people only queue for Shatter." If players made efforts together to actively queue Secure or Seize, they can and have made it happen.