why is the cast of the Scions so...elvish now.

[hydralus]

FFXIV has final-fantasied so hard that it's made white haired characters the norm and dark/hazel haired characters exotic.

[Cilia]

Part of the "problem" is that the Scions are an Eorzean-based organization, and all of the new races introduced post-2.0 hail from outside of Eorzea and so would logically be fewer in number there. We often get "exotic" characters that serve in supporting roles during expansions (Yugiri, Gosetsu, the Xaela tribes, Lyna, etc.) but the core Scions still being main characters means there's not really any openings. They could introduce new main characters without benching or killing any of the already-present Scions, but that could lead to a cast bloat I'd rather not see.

Taking that into account though... I'm disappointed there's no "main" Roegadyn Scion, and other than Tataru (who is more of a supporting character, especially in Shadowbringers and Endwalker) no main Lalafell Scion either after Papalymo's passing. Also wish they'd give more Viera and Hrothgar characters speaking roles, but alas.

... but why so many Elezen (elves)? Well, everybody loves elves. I mean, just look at the myriad variety of them in Dungeons and Dragons! (Apropos of which, it also has rough analogues to both Viera and Hrothgar... but both were only introduced recently and are nowhere near as popular as elves.)

FFXIV has final-fantasied so hard that it's made white haired characters the norm and dark/hazel haired characters exotic.

I think part of it is that white hair stands out and so is used to denote someone "important" or "mystical," and part that white hair makes color coordinating an outfit way easier.