Because you need to hold it in your hand which would invalidate most magus subclasses as they either need a free hand or hold a 2-handed weapon!
You can start with a general feat for Armor Profficiency and later down the road take the sentinel archetype (i know it's a sacrifice but i believe it's worth it) and retrain armor profficiency to Toughness
You can start with a general feat for Armor Profficiency and later down the road take the sentinel archetype (i know it's a sacrifice but i believe it's worth it) and retrain armor profficiency to Toughness