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

[Aerlana]

they probably didnt change the core of the code from v1, for a game with different objectivs than v1.
They have hard time so to developp anything.

For the lack of fun, 12, 17, 24 month wouldnt change anything, they dont have good game designers for fight content...
They have the High Level PvE team (that provide omega, primals, 24-raids etc)
the diadem/eureka team (yes... the same team did the 3 fails...)
The palace of the dead team (FUN FOUND!)

The first group wont specifically try to find the fun, but interesting fight mainly. (If the fight is good, interesting, the fun will be there)
The second and third have to make the content fun, but only one of the two team get some good game designers...

[ReplicaX]

the diadem/eureka team (yes... the same team did the 3 fails...)
The palace of the dead team (FUN FOUND!)

Eureka:

• Map design alone is a breath of fresh air (many ppl still don't care)
• Players are now optimizing NM spawning, initial first impression salt of just chaining non-NM pop mobs is now irrelevant unless you just want to solo everything.
• 20 NMs specifically designed for the content and many of them are above S Rank designed hunts and we haven't even gotten to the higher NMs.
• Initial Salt they copied XI, yet it is trace elements of XI and newer content but still with the base grind relic that has been staple FF tradition.

Palace of the Dead:
Content with even more elements of XI's Nyzul Isle only watered down for XIV players. Even the RNG shard trade in is a watered down Nyzul turn in.

Hate to bring some reality in (definitely not defending SE, as the 4yrs of base tome grind is old as hell), but between the initial outcries of Eureka with players just mindlessly grinding trash mobs and the "this is not XI" hate. Many aspects folks consider fun have more elements of XI in them than Eureka. I wouldn't be surprised if the new deep dungeon had elements of XI Salvage or other XI content that was highly successful modernized to fit the XIV formula.