Why is diadem a duty?

[NekoMataMata]

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.

Maybe not open world but what's to stop them from putting it in a non-duty instance?

[Deveryn]

Maybe not open world but what's to stop them from putting it in a non-duty instance?

What's to stop them? It's already in place and not worth migrating just so people can queue something else. You go to diadem to do serious gathering, not pass a little time in between queues. There are plenty of other places to gather while you wait for your spot.

[Fawkes]

Maybe not open world but what's to stop them from putting it in a non-duty instance?

I assume they do it that way because they expect it to be popular at launch and don't want people hogging it up AFK. In a duty you'll get kicked after a few minutes of being AFK. If they just made it regular zone with instances then everyone would AFK there and all the instances would be packed full all the time (at launch, at least), which then also causes other problems like making it hard for people to go into their houses because of how the instance server works.