Any sensible soldier will want their weapons made of the best material available. Bronze is what humans have as their best weapon metal, so that's what they mass produce and sell.
But not
everything will necessarily be bronze. If we were talking about the standard issue that a kingdom supplies to its military, then I would agree with what you're saying, but we're talking about an adventurer buying from shops in the towns s/he comes across during travels. Copper or iron weapons might be sold because they are cheaper. Bronze being the standard is fine by me, but I'm wondering why everything is exclusively bronze (except for kobold/ gobbo stuff) when it wasn't like that in previous versions; at least, it wasn't in the ones that I adventured in.
Bandits however, are known to carry all sorts of metal in their weapons. Even more if you mod your raws and then generate a world.
Not bandits, they all carry bronze for me, but I've seen steel, copper, and iron on kobolds, and from what I hear, goblins use them too.
Personally, I'm perfectly fine with ending up with a ☼copper spear☼. It makes things more interesting, because then getting adequate equipment becomes a part of strategy as well. "Should I risk trying to snatch that spearman's high-quality iron spear from him right away, or should I let my companions take care of him while I fight someone less dangerous?"
Just started up a world in the new version and it seems there is much more variety in materials for weapons + armour.
That's good to hear. Then maybe the all-bronze stuff was a placeholder. I'll have to update.
EDIT:
Hey, Toady? What does this mean:
(*)changed metal use for humans/gobs/kobs via existing raw tags
There's a little more detail in file changes.txt. Humans have smelted ore and mined stone now, so they have copper, iron, silver, etc. and similarly for the others. It had been randomly choosing metals for kobold weapons in particular, which was okay in the sense that it mimicked thievery perhaps, but we want to move over to something more genuine before too long, so the kobs are on placeholder smelting for the time being.
From the thread announcing DF version 31.18. I wonder if this means that steel will show up in stores now.