dave2008
Legend
It's mostly about consistency in naming. When I hear gold, silver, copper, iron, adamantine, I get a "one of these things is not like the others" disconnect. I guess the fact that the version with bronze and brass has two alloys makes it a group of three and a group of two in my mind, instead of a group of four and one outlier.
I get that, but for me Adamant is such a D&D staple that it doesn't sound out of place to me. In fact, I would find it out odd if it wasn't included (if you are also including iron, steel, mithril, etc.).