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.

[FJerome]

What makes it especially ironic is that the backstory for the Starlight Festival shows that the holiday ORIGINATED in Ishgard. A devastating dragon attack produced record numbers of orphans, and in spite of being expressly forbidden from doing so, Ishgardian soldiers would smuggle them into their barracks to shelter and feed them, wrapping them in their red uniforms to hide them whenever superiors dropped by. As adults, these orphans remembered their benefactors' kindness, and performed a kindness in turn by dressing in red garb and distributing gifts to children themselves, thus starting the tradition that lead to the holiday as it is now known.

It certainly wouldn't be inappropriate to have dressed up Ishgard for the holiday, even if none of the actual holiday events took place there!

I think you can hand-wave it because the origin story was specifically about knights disobeying orders, which the Church would frown upon, so Starlight celebrations in Isghard evolved to take place entirely behind closed doors. So even now, with the Church's power broken, celebrations are still small and private but more now because that's just how Isghardians have always done it.

[Belhi]

I don't know if it can really be relied on as a source but dream Amyeric says that the festival is still held in Ishgard to this day. It can be handwaved away by simply saying though that in Ishgard it's a much more private indoor affair. After all not many people are going to be gathering outside on the coldest nights to celebrate it and constant war would probably incline Ishgard to keeping most festivals outside tournaments low key. Even then they seem to be rather invitation only.

[Airias]

Not even Fortemps Manor has any decorations up, so it's not even an indoor affair. Granted, no one visits the Manor when not on a quest, so decorating it seems a bit futile.

Seeing as last year's Starlight Celebration was all about the Ishgardian history of it, and this year we even get told by dream-Aymeric how it is an important and beloved holiday in Ishgard, I just had expected a little bit of festivities - at least the kind of decoration the other cities have. Ishgard is my favourite city and seeing it empty while the rest of Eorzea is celebrating Ishgards own festival, is just disappointing.