You’re spot-on with a lot of things here, at least according to what I read 
Bronze was expensive and difficult to manufacture, since it’s an alloy of two metals. Iron only requires one ore plus coal / carbon, so it’s cheaper and easier to manufacture. Using iron let you spam weapons and tools at much higher rate, so to speak.
Interestingly, I read bronze was a superior metal to iron and early steel. It was more durable and held a sharper edge (I heard they used brass razors way into Middle Ages). It’s still inferior to true steel, of course, but that kind of steel was developed pretty late. The biggest drawback of bronze weapons was that it bent and deformed easily - but even then, you could simply smelt a bent sword and cast it back into the right shape.
Also, thanks for the link - I’ve had some thoughts on Athas materials myself, so it’d be great to compare notes
@redking SeruZmaj gave some really good answers in your thread. I agree with a lot of what they say: There probably are rich veins out there, they’re just out of reach. Also, a lot of knowledge was lost. Entire races have been purged, and even those that were not have lost much. Also, I think a lot of mines were abandoned. Wasn’t the Tyr mine discovered several centuries after Kalak settled down?
I also wonder if the small size of the setting has anything to do with it. I once read the whole explored area is the size of California, Nevada, Arizona and Utah. That’s just a tiny slice of the planet. Is metal absent on Athas, or just in the known part of it?
Perhaps there still are rich ore veins, but they’re just too far away? Maybe the Tablelands and surrounding areas were always poor in ores? Prospecting for metal in faraway lands is certainly impractical, considering how tough it is to travel from one city to another on a road. It might still be out there, just… Out of reach?