Generally speaking, its because:
1) the pulled pack size is pathetically small and not worth interruption you dps rotation.
2) not using it on a boss in the hopes of a not pathetically too small pull that an aoe lb is worth more on. which would overlap maybe 1/8 earning of a full bar
3) playing the is it worth it to wait game on the final boss. If the lb bar hits a full 2 at 17% in a dungeon your dps have bigger problems than remembering to use the lb at all.