It's obviously not the only reason, but it's a pretty big one I imagine.
And because it seems they decided pretty early on that mostly no headgear was the way to go they went a bit nuts designing some of the hairstyles for Viera, several of them have so much volume on the sides and in the back I'm not even sure you can properly cull them or squish them down to even look nice under something as simple as a bandana without ruining the hairstyle itself or causing the headgear texture to stretch to a ridiculous degree.
Hrothgar has the horned face which could cause problems (Do you remove it temporarily? Let it clip through the front like Au Ra horns?) but otherwise look much more hopeful for some wearable hats than Viera honestly, but they're in their own unique predicament with the Fantasia hair change situation and really only 2 hairstyles per face.
They made a big mess creating these races to a very narrow, specific vision seemingly without taking into account how players would take being left on the sidelines for every future hairstyle or headgear piece, at least until people kept being vocal about it and they gave a vague "it isn't the best situation and we will continue to work towards alleviating these issues and making them better in the future and finding ways to make things work".
I'm somewhat hopeful they'll do what they said they would but it feels more like PR speak and we'll get thrown a few scraps at some point but no major changes. They really designed themselves into a corner here so it'll be difficult to try and make these races equal to the others (and I don't think that was ever their intent either) without fundamentally reworking them. It seems like they think the "uniqueness" makes up for the lack of everything else.
But after one and a half year I've become pretty cynical so I'd
love for them to make me eat my words and prove me wrong.
