By infiltrating Vinland, Eivor is essentially going under disguise. Gorm and his crew have wreaked havoc on the region, and as such, even looking enough like a Norse or foreigner from afar could get her killed. Eivors death would cause a time paradox and ruin the continuity of the games, so a happy medium had to be achieved. From a game design standpoint, it would also make sense that the developers wanted to further challenge players who unlocked Vinland. Ubisoft wanted the hidden map to feel unique and therefore sought to create weapons and armor that couldn't be found or used anywhere else. It is a common gameplay design element; they're not the first to do it.