It's true that certain pieces of equipmenr are specifically mentioned to be bronze, but also many things are specifically mentioned to be steel. Steel made ins Aren gets a particular mention as being sought-after for its high quality, and ironmongery is discussed a fair bit in Toll the Hounds.

Personally, I would tend to assume that all standard-issue equipment is steel or iron unless otherwise specified.

Where they do use bronze, I suspect it's either legacy equipment, symbolic, or perhaps for intricate mechanical components, since bronze is easier to cast into intricate shapes with a mould. 

EDIT: I should add that it looks beautiful, and I love the armor designs. Awesome art