There's a limited number of 'slots' for armour models. In K1 it goes from A to I, with slot A being used for clothing. It's an attribute of the NPC's appearance, not the item; Carth's appearance A is his orange jacket, Trask's appearance A is the republic uniform etc.
The workaround is to use the Disguise item property, which completely replaces a character's model. That's how the sith trooper outfit or the mandalorian armour works.
For the player character they're only useful for full-body armours like that because the head/skintone/dark-side-transition doesn't carry over; it's one model whoever equips it.
For party members, though, you could make it work. A clothing item that only Bastila can equip, with a Disguise that replaces her model with 'civilian outfit Bastila'. There's probably already a mod that does that with her 'slave' costume.