अश्वव्यूहो वर्मिणामुरस्यं शुद्धानां कक्षपक्षाविति ॥ कZ_१०.५.३५ ॥
aśvavyūho varmiṇām urasyaṃ śuddhānāṃ kakṣapakṣāv iti
In the cavalry array, the armored horsemen form the breast/front; the unarmored (or lighter) form the shoulders and wings.
Armor increases survivability in direct contact, so it is assigned to the most exposed, collision-prone sector.