It's understandable that one would use the most popular races for advertising from a monetary standpoint. If I were trying to sell FFXIV to a group of individuals, I would most definitely put Miqo'te and Au Ra on a stand.

The fact of the matter is, though, that we're talking about an in-game event that requires no real money transaction (outside of an active subscription) to participate in. Giving some screen time to some of the less popular races will not make players opt out of the event, nor would it make people not want to play/purchase the game itself.

Furthermore, in a game with so much focus on lore, especially with the many different races and how they interact with one another, it almost feels self-defeating to give focus on just a handful of races due to their popularity among players. Hell, one could even argue that the popularity among races could actually fluctuate if more attention was given to lesser-played races.