Why is diadem a duty?

[Deveryn]

There are so many side system Duties that just need to be open world content and use the actual environments created off of MSQ. It's very frustrating, but I do understand it to some extent - Part of the reason is the ability to make them cross-DC.

Open world wouldn't have the same flexibility that the duty version offers. Aside from opening up to the whole DC, you also have things like the duty action and limit bar which just wouldn't exist in an open world setting. It allows gatherers to level up without being fully progressed on main story.