Can someone from devopment explain why Eureka was delayed for so long?

[Squintina]

party
Probably a big chunk of the time was spent on the ability to party and change parties in an instance.

It might seem like a small thing, and one we take for granted, but that was probably a lot of work to figure out.

Because partying inside Eureka is not at all the same party system as outside. It looks the same, but it isn't.

How do I know that?

If you make a premade party outside of Eureka and queue, sure it looks like all it did was take that same party and applied it inside the instance.
However if you change your party members inside (a person leaves, other people join, etc..), that same original party in Kugane remains.
That implies the party system is different between the two

Also anyone who's running ACT will notice that it's having some difficulty telling "party" members alike. Instead it's treating everyone in the whole field equally (leading to some pretty gigantic logs). That also implies that the partying inside is a different system than usual, that was just skinned to look exactly like the old one.

NM

There are 20 datamined fates, so 20 NM. Many of these have unique features (which if you're lagging might be hard to see but those of us who aren't do see them).

Some of them have stack markers, others player-target AOE, others call in reinforcements, etc..

Having fates that trigger based on kills also probably took some development time. There are no fates elsewhere that do so.
Other fates trigger on: timer, fate chain, and NPC talking.

Weather and time based monsters
Typhoon sprites show up in gales
Rain sprites show up during rain
Undead show up at night

I don't know if they've done anything like that before, so that might have taken development

Other things we haven't discovered yet?
We still haven't explored the level 20+ areas. They might have their own small quirks that required development time.
After all, we didn't know about the "buff faeries" as they weren't mentioned anywhere, we had to discover them. Maybe there are other things that we still need to discover, perhaps that are better hidden.