Copper is one of those materials that due to its poor performance in extended combat and its extremely low value (it's worth the same as wood for gods sake,) comes off as useful only for cheap tools for would-be settlers and training for aspiring smiths. "It's better than nothing, I guess." Is probably a sentiment echoed across DF worlds with mild disappointment and during times of war, possibly even dread.
Any non-military role copper can do, wood can do just as well and for fewer resources and a fraction of the weight. For weapons and armor, literally any other martial-grade metal is better except when compared to silver, where it's an inferior mace or hammer metal but a better edged weapon metal. In DF it's just not a good material for anything beyond self defense against wild animals and as ammo or skill training, and even then anything with enough mass behind it is gonna shrug off even well-made copper weapons; hell, something big enough wearing regular cloth and leather clothes can put serious wear on iron and bronze armaments, I don't even want to think how badly it'd ruin copper weapons.
It just comes off as a material nobody should want to field
ever if they can at all get anything more effective, unless they have some serious pride in the local coppersmiths' workmanship, and even then that workmanship will only do so much to compensate for material weakness against the beasts elves and goblins can often bring to bear. And not only that, just look at the elves - they could have the finest wood arms and armor ever made, but it's still wood up against metal, and it'll come apart under moderate punishment. Likewise you may have the best copper armor ever, but a guy with an iron sword or axe isn't going to give two shits about that; his weapon is objectively better than your armor, forcing a reliance on skill over equipment to be able to compensate because such a weak material just can't be trusted to withstand too much punishment, no matter how well made it is.
Besides, steel with worth its weight in gold, and iron its weight in silver. Granted getting steel to work requires trading with the dwarves for it, but still.
That's being rich and extravagant, not copper. And this is a place with people looking to strike it rich in creating a new trade route, right?