Part of it obviously comes down to artistic licence and developer constraints. This may offer the explanation you're looking for though: [url=https://www.halowaypoint.com/en-us/universe/species/jiralhanae][i]"Upon integration into the original Covenant body, the Jiralhanae held a number of positions. Some of these jobs were simply an extension of their own native, clan-based organizational structure, though it became more formalized while under the banner of the Covenant. Roles included Chieftains, Captains, Stalkers, Majors, and Minors, all splintered into a number of varieties based on skein, functional and organizational placement, and the point it appeared during Covenant history."[/i][/url]

Idk where I heard it but I thought that the unshaven in armored Brutes were specific to Tartarus' pack. Specially trained to be fearless without armor. But I don't know.