Q: With the two new races' unique heads, how will their headgear work?
A: It won't be displayed, because those face parts wouldn't work in FFXIV's system if headgear were involved. Even Viera ears would've had to be moved down to where Miqo'te ears are. It would've been meaningless if you couldn't go all-out creating characters from these new races, so we decided that helmets wouldn't be necessary. You can equip them, but they won't be displayed. However, things that replace the head are OK, like the snowman head, fat chocobo head, etc. No glasses. Basically, the default is that it won't be shown, and only things that replace the head are ok. We'll make more things compatible over time, bit by bit, but they won't be compatible at launch.
This is a translated Q&A from March 2019.
https://www.gamer.ne.jp/news/201903240006/
It was after this interview that Yoshida learned that some of his staff had done off the clock work making a limited number of pieces of headgear work for viera and hrothgar work. But the initial implementation was always to be that the only pieces that would be display are those that replace the entire head. They did indeed make more available. But it shouldn't really be a surprise that with all the other stresses on the work environment over the past year that adding additional helms to the races was not considered a high priority. Playable content always takes priority over something like this where the game works even if you can't wear hats, especially when this is an issue that effects less than 10% of the playerbase.