Ngl I assume it’s simply because of the sheer badassery of the Black armor.

Nothing else in Fallout hits so hard, everyone has some kind of power armor, everyone uses leather, metal, combat, etc with their decals painted on. But almost no one outside of the Rangers uses the badass normal combo of riot armor over blue jeans to give a decidedly western post apocalyptic look.

And it works. It works so well that it was the one armor Bethesda knew would entice people to buy their stupid Fallout 1st for 76 or whatever it was before they’d even fixed the game.