Why is there no Starlight Celebration in Ishgard?

[Cilia]

If anything, it would be the real "excuse", seeing as SE has no qualms locking ARR only players and newbies out of experiencing the zones as a whole. They're already missing out on the entire scenery, story and gameplay, so claiming missing out on a few additional decorations that aren't even unique to the area as reason not to do it has very little substance to it.
If it's indeed intentional, the "reason" would be cutting corners here - it's one less area to work with, plain and simple. And indeed, far from a first in business - at least they aren't selling the celebration as DLCs per city.

Newbies not getting to Ishgard makes sense within the context of the story - the nation is isolationist and it's only because Lord Edmont is willing to let you become a ward of House Fortemps that you're allowed entry. (NPC dialogue says other adventurers are as well for their assistance during the Steps of Faith.)

There will never be DLC for this game unless you consider expansions to be as such. That's what the subscription fee is for.

There is no real reason they didn't add Starlight decos to Ishgard, but personally I'd rather they cut corners on that and finish Stormblood on time (or early!) than put some decos up in Ishgard for a few weeks.

In other words: unpleasable fanbase is unpleasable. Nothing to see here, move along.