Lately, there has been a lack of (Hard) difficulty dungeons, so that would contribute to this as well.

A Realm Reborn: 10 (Hard) Dungeons
Heavensward: 5 (Hard) Dungeons
Stormblood: 2 (Hard) Dungeons
Shadowbringers: 0 (Hard) Dungeons
Endwalker: 0 (Hard) Dungeons