Changeling does give reasons why the fae hate iron.
Yeah, but this isn't Changeling. The Raksha don't actually work the way either of the Changelings do, though they have shared elements from both. As a result, it's nonsensical that they'd be harmed by iron for "breaking contract" with it, or because it's "boring" (which it isn't).
The former has some basis, in that there could be a really influential
story about a great raksha king who was poisoned by a rampant beast of iron, and so all raksha must now fear iron, but a) there isn't, and b) that doesn't make a tremendous amount of sense for raksha.
You could take the basic idea of "banality" and tie it in with Autochthon, and say he made iron as a tool for lesser races to fight off the raksha, but a) that isn't how it happened, and b) it doesn't make a huge amount of sense because the threat facing Creation before the Primordial War was the
Primordials, not the raksha.
You could say it represents stability and the pole of Earth, but a) it doesn't do that anywhere else, like in thaumaturgy or sorcery or backstory or themes and b) in that case, why doesn't the anima or Elemental Bolt or an Earth-aspected Terrestrial do aggravated damage - and why doesn't Jade, which is
actually the thing that represents stability and the pole of Earth.
Hell, gold and silver are both more "stable" than iron, and they're also
both tied to gods who fuck up the raksha in different ways, though Luna has a better claim to it just because it's what her Exalts do and it's what silver actually does in mythology - including mythology that Exalted actively tries to draw from, as opposed to tired old Eurofae tropes.
I would wager that iron was picked because iron is common, thus making raksha power countered by something even a dirt-poor commoner from the Threshold might have on them, which feeds into the general "fuck you raksha" narrative the game has in places. Jade, by contrast, is precious and already magical.
So make it silver. That's precious, but it's not "a sorcerer will round up your village and have everyone turn over their inherited jade talismans on pain of death so he can use it as ore to smelt an animated jade-alloy statue to crush his rivals" precious, or as rare as any magical material, and thanks to the Guild it's even more common in the Threshold than you might expect, perhaps as a way of helping to keep raksha from their clients (and thereby driving up the value of the slaves they sell to the fae). It's something that a mundane blacksmith can edge weapons with for when the Wyldstorms are blowing and the season for rakshasha arrives. It's something that can coincidentally save a human's life when their grandfather's silver bracelet burns the paw of the raksha trying to drag them away in the night. It's not something that any advanced society is going to quickly reforge into steel, making it useless somehow because iron+coal = not stable, not good. It's something that actually makes sense in the setting.
Maybe because Raksha are inherently whimsical and irrational? Because they occasionally pull (not necessarily intentionally) the tropes from works of fiction and apply them to themselves, and at some point the Raksha pulled a trope that has no relation to Creation whatsoever and is still bound by it?
Right, see, when I ask the question "why has this concept just been lazily copy-pasted into the setting from an inappropriate source, with no justification", I kind of hope the answer won't be "because we lazily copy-pasted it into the setting from an inappropriate source, with no justification,
in-setting". That's the sort of answer which requires really, really clever writing for me to accept it, and the raksha don't even hit one rank of "really", even with Moran's last-minute rewrite.
