It is fairly simply. There are two hypes of heavy armor in game. Aim gear and strength gear. The two types have two different aesthetics. Aim gear is very soldier-esque. While strength gear is cosmetically designed for force users.
Companions like Qyzen would look fairly odd in a robe and would more appropriately wear Trooper looking gear. Instead of making another class of armor for each slot, head, chest, legs, feet, bracers, that looked like Trooper/Bounty Hunter gear but used strength, Bioware decided change one equipment slot, weapons, to use aim.