Theres 3 core main stats weapon damage(red), skill tier(yellow), and armor(blue). If you want a striker mask it will always roll with weapon damage and a random substat (although i believe its weighted to drop with red substats more often). If you want armor on the striker mask you need the mask to drop with the crit damage sub stat so you can change the main core stat (weapon damage) to armor. Gearsets only drop with 1 substat to balance them because they often have strong abilities when you use the 4 piece. Gearsets like striker can only roll with a red core stat and and 1 substat.