An instance started that I was queued for but queue did not pop for me

[ItMe]

I highly doubt that all 24 players queued for that raid specifically before me, so the game obviously threw a bunch of rouletters with a few that may have queued for Royal City of Rabanastre when there was someone who was queued for it who needed a clear who was not thrown in with them.

If Galarian Articuno needed a clear and she was queued for it, why did it not throw her in instead of one of the people doing roulettes?

Because that's not something the game tracks.
Everyone's "ticket" into the raid is identical.
More than 24 people qued and you just didn't have RNGesus on your side to get you picked.