Just like the raiders in Fallout it's a strict dress code for bandits
Side note wonder if they take equipment from fallen enemies? In Fallout 3 I once run into raiders in power armor, they had taken a enclave outpost and used their armor, some had armor other had helmets.
To the Fallout 3 Raider's credit, there was multiple types of Raider armor, but same issue. I don't really have anything against the "Respawning" shallow enemies, it's just that it's so odd seeing every bandit, even supposedly rival factions, in the same attire. At least if they gave them territorial-based "Bandit armor" skins, it would make more sense, and at least remove one layer to the disbelief, since it creates a semblance of some organization. I mean, Crips and Bloods don't wear the same stuff, right?
I'd still rather see "General Issue" bandits used as Overworld populators, rather than dungeon populators, every dungeon involving NPC's should be hand-crafted to the finest detail, to give them that "Lived in" feel. Animal/Daedra caves don't need that kind of detail.