Why doesn't WvW XP count towards Mastery??
Its not even just the xp to unlock first tier of crafting legendaries. Suppose you get wvw xp count toward mastery and get legendary going. Then what? The whole process of collecting pieces in each tier involves running around the PvE maps, collecting stuff, doing event, killing champs. You must do fractals. You need to grind in new HoT maps for specific currency that drops only in that single map. You will be doing this all for many months so no time for wvw anyway.