The notion that bronze was always superior to 'iron' tends to be supposed on the basis of the properties of pure iron. No such thing existed in the middle ages. Iron was in all cases polluted with various slag inclusions and often did have some amount of carbon in it, either as leftovers from the smelting process or added in later through case hardening.
Medieval iron is more accurately called wrought iron, its been said to be roughly comparable to modern mild steel, but quality, carbon content and quite importantly also the degree of homogeneity make this a pretty wide estimate.
Bronze is also a bit of a tricky thing, since freshly cast bronze suited for armor and weapons is still fairly soft, but was always work-hardened with various levels of skill. Composition and consistency varied as well, so statements of bronze or iron being better than the other in general can be safely dismissed.
The reasons I eventually chose not to include it as a weapon/armor grade material are the following: DF doens't give me any way of setting the prevalence of a certain metal. Either you don't get bronze items, or bronze items out the wazoo. The later, I felt, gave the wrong feeling for the medieval era.
There is also the thing that bronze-working is a different craft than iron/steel working. The process is quite different, and bronze armor and weapons had some much more stringent requirements on the quality of the work and materials than say, a bronze or copper kettle.
Lastly, material in large part dictates what shapes of weapon and armor your going to make. Long, thin swords or complex plate armor would simply not work if made from bronze (this goes for things like iron estocs as well to a lesser degree). Steel and even wrought iron have a very useful elasticity (read: the ability to deform and spring back again) that bronze doesn't have. Springiness like that tends to be more important to making durable, lightweight weapons and armor than hardness.
Adding the appropriate tags back on bronze should be fairly trivial, and *might* not even require a new save. What I have not isn't perfect, I do want to make it a bit more reasonable once I get around to working on the metalworking mod again. What have planned should give me more control over what things can be produced with specific metals.
Edit: In response to the cloth density thing, that was added to make gambesons not weight a ton. Seems I missed it on my comment tour, good find and thanks for pointing that one out!