With Lyse it's because female Highlanders didn't exist at the time she was designed, and 1.0 clearly wasn't thinking super hard about Ala Mhigo or its presentation anyway. The pair of groups in 1.0 that merged to become the Scions did have a bit more skin color diversity; the Path of the Twelve had the darker-skinned Hermit of the Wood, Percevains and Satzfloh, at least two of which did have non-white hair colors, while the Circle of Knowing had Louisoix and Rammbroes (who's now with the Sons of Saint Coinach). Lot of white-haired people, but that was partially credited to a complement of old people.
With everyone else it probably just mostly comes down to individual design sensibilities; this design makes sense for what this character's history and personality is. And then once they had the Scions largely out of stitching together existing characters that made sense and were fairly prominent, nobody really raised 'hey, this lineup is super white'--or if they did, they didn't have the capacity to do anything about it, because all these characters were already there.