But that doesn't really answer the question, though. Plenty of humanoids lack protective shells, scales, etc., too...but oh heavens no, they aren't allowed to cover up with metal for protection. But if they're lacking claws and fangs, metal is acceptable?
It's just so weird and arbitrary; I feel like it should be all-or-nothing. For my part, I'd prefer druids who won't use metal of any kind, not even coins for currency...they fight with spears and bows, their armor is made from chitin and hides, they trade in pearls and pelts, etc. Much more evocative. If you're going to put in a restriction "for flavor,"
commit to it.
The real interesting part about this is that it implies that Druids didn't exist until after the end of the stone age, where there was any kind of metal to prohibit. Now this is just my headcanon at this point, I have no information to work with, other than the existence of Druid and Priest-adjacent classes in D&D's past, like Angakok's (wait no, those are Wizards), Shamans, Shamani, Spirit Shamans, Shugenja, Witch Doctors, etc..
If you consider that divine powers come from pacts with deities, not dissimilar to the pacts a Warlock makes (there's a major religion in our world that basically starts with a pact someone made with a deity, so I find this credible, especially since there is a story where a pesky magician subverts that pact to gain the power of foresight), then perhaps these pacts are "renegotiated" over time.
In the early days, primal powers didn't have any metal to worry about, but once the bronze age began, maybe when the proto-Druids asked for powers, the primal entities (whatever they are) were like, "hey, you know, I don't mind so much if you harvest the hides and scales of living creatures for defense, but I'm not so keen on this bronze stuff". Or maybe this occurred when the iron age began in earnest. Maybe there's an expiration date on Druid powers, and every so often, the Hierophant has to hammer out a new pact.
This could neatly explain edition changes to their powers, and how Druids snuck in "agricultural and hunting tools" to their weapon list in the first place.
What we're seeing in 5e could be the result of those negotiations going on
as we speak, thus explaining that while there is current no actual ban against wearing metal armor, no Druid is going to risk it for fear of torking off the primal powers- and maybe in a few years, those entities will relent and say "sure, fine, you can wear bronze, or even iron armor, but none of this adamantine stuff!"