The answer is almost always money.

Compared to many other MMOs, ESO is very generous. For a minimal sum you can unlock the basic hair pack which includes many styles, and change them whenever you like without any further cost.

Nice.

The value of individual hairstyles beyond that is questionable and obviously varies by taste.

I think I've only ever bought the one - the Dominion War-Locks - because it looks quintessentially Elven.

If players aren't buying new ones from the store then they won't make new ones.

They are extremely light on hairstyles and especially facial hair styles, but even if anyone knew of any that were unreleased, there's a policy against posting them: https://forums.elderscrollsonline.com/en/discussion/620737/policy-change-for-posting-rumors-datamined-content-unofficial-unreleased-content-on-the-forum#latest

But I will even say that the motif rates have also slowed considerably. In 2019, we got 10 motifs for the Elsweyr year (Coldsnap and Meridian in Q1, Anequina, Pellitine, and Sunspire with the Chapter, Stags of Z'en and Moongrave Fane in Q3, and Dragonguard, Shield of Senchal, and New Moon Priest with the Q4 DLC). 2023 didn't give us a Q3 dungeon pack or Q4 DLC zone, so we only got 5 (Blessed Inheritor and Scribes of Mora with the dungeons, and Clan Dreamcarver, Dead Keeper, and Kindred's Concord with the zone). We also slowed the release rate, as most zones dropped one motif on release and then the second (and trial) motifs released later, but since Galen we haven't gotten any motifs dropping on the release of the zone anymore.

All we can do is hope that the art team looks into releasing more.