Probably mostly just balancing reasons and a different RPG based direction this game went in.
I don’t think there’s a lore reason yet. In many ways, people of the War Seas are much more advanced than the Seven Seas, with imbuing and creating unique spells, as well as the immense diversity of magic in general. It could just be a different mentality to using magic.
The sea clusters are half a world away, so there’s almost no overlap or knowledge of each other, aside from the exceedingly rare traveler crossing the Dark Seas.
Fighters in the war seas can do a lot of things that people of the 7 seas weren’t able to, such as the many unique uses of their magic aura (imbuing any fighting style or weapon with magic/FS). While it may have been more common to see a magic user in the 7 seas have all three of their minds unlocked in addition to a mastered fighting style and weapons, war seas people just have so many more applications of their power that not having 3 minds isn’t a setback