Trying to think of justifications for why Forsworn/Reachfolk equipment is like that. Being made of weak leather, bone, & wood while everyone else around them uses steel.
Just saying they don't or that they're primitive doesn't work when no explanation is given & there is nothing stopping them from learning it from more advanced neighbours.
ESO's Iron Orcs are actually one of the greatest flaws to it in how they do have a metallurgy narrative. The Iron Orcs are their southern neighbour who believe its blasphemy to break rock & mine, but did so anyway in order to compete with more advanced enemies. So why hasn't the Reachfolk done so?
My first justification is that for them, leather & bone isn't inherently weaker than steel. Reachfolk uphold Hircine as one of their most important gods. So by dedicating their hunts to him, they can receive blessings on items made through hunted materials. Reachfolk equipment can therefore match steel through divine blessings.
This is combined with straight advanced leatherworking skills since they do it so much.
My next justification is that they do indeed have a cultural stigma against mining, but for obvious practical reasons. If you tunnel deep in the Reach (at least the eastern part), what you're going to find is dangerous Dwemer ruins, and Blackreach full of Falmer. They even have old stories of vampires coming from below to plague them.
This actually brought about one of the only exceptions to their disuse of forging. The Wayward Guardians started using silver & scavenged dwemer metal in response to the extreme threat of the Grey Hosts vampires & werewolves. This practice did not last however.
Mining is just inherently dangerous and not worth it while Hircine enables leather.
Finally is that the easterns reach's mineral wealth just isn't in martially useful metals anyway. Sure there is iron & such, but the majority of mines available are for silver. Cool if you want to get jewelry for your beloved, but currency is mainly a foreign tool to them. The clans have more use for directly trading & bartering goods. Not like any unifying state of reachfolk can standardise the currency (not counting foreign occupiers).
Some of the Reachfolks few experiences with mining is when colonisers push them into working the dangerous mines for them. A deeply negative connection.
In conclusion, the Reachfolk could mine if they wanted to, but they don't want to take part in the dangerous, miserable work so long as they have a more culturally appropriate alternative.
Would they benefit more from using metalwork to fight invaders? Probably, but how much would it cost them as a people. Bleeding into the ground, turning from Hircines sacred hunts, and using the ways of the Enemy.
