Honestly, I think the main reason Eureka Orthos is so dead is just that it's out of the way.

With Palace of the Dead and Heaven on High, the entrance is a few feet from the nearest aetheryte. You can zone in and queue in about 2-3 seconds, plus whatever the loading time was to teleport to the aetheryte. Contrast that with Eureka Orthos, where you have to teleport to Mor Dhona, fly over to Syrcus Tower, click-enter the Syrcus Tower map, which causes another loading screen, and then walk for about 5-6 seconds before you can talk to the NPC to enter the Deep Dungeon itself.

Trusts also clearly cut into it as a resource for EXP Grinding, since they're available on demand and are effectively effortless content, whereas Deep Dungeons require some degree of situational awareness even in matched parties.

My point being that they should be a bit more convenient to enter, since their Path of Least Resistance alternative is too.