It was also available for centuries before plate was, so a great many more people had the training to make it.
actually, no it wasn't. think about it, it is a fairly common misconception. making a sheet of metal more difficult than making interlocking rings? greeks used bronze to make a cuirass which is too soft to make into decent chain armor. carbon fiber vests were used before that and dubbed linothorax, which is quite different than medieval padded cloth.
First, while Bronze was used in brestplates and far earlier than the medieval period, iron and steel, which replaced bronze due to availability, was a good deal harder to work, as making the large sheet required needs fairly consistent temperatures across the entire surface, which was much tricker due to the higher temps that iron is worked at. Further, once the rings are made, much less skill is needed to assemble them, meaning that much of the labor can be done by apprentices, rather than the expert armorsmith. (I've even come across sources suggesting that whole villages participated in the weaving of mail, but I don't know how reliable those are.) Also, a flaw in plate armor ruins the entire piece, while a few bad links in a coat of mail can easily be replaced.
Taking it even further, mail is lighter and more flexible than plate, meaning that the arms and legs, which in classical times were largely unprotected, going by museum pieces, could be covered at a small penalty in flexibility.
i avoided saying iron or steel plate for a reason, not that i don't think that steel 'trauma' plates were used, but that there is a debate on whether or not steel was used for such in prehistoric times. any way you cut it though plate armor came before chain, after all not only is bronze easier to work with, but protective plates are pretty intuitive compared to chain.
All you need is to have learned how to draw the metal into wire.
The making of wire, and hence the creation of all manner of convenient things like, oh, nails, is actually much more complicated than you'd ever think, so half the problem (or, maybe even 90%) of historical chainmail was getting workable rings. Early nails were a real hassle, too. Obviously, the current DF model is a mix of technologies and techniques that, in typical fantasy fashion, has been mushed together, so the only way you can promote someone using an inferior technology is either through arbitrary means or by giving some ahistorical benefit to particular tools and techniques.
throw in folding methods for sheets and you get a viable method for creating steel plate armor well before one can make steel wire consistently, and you end up with a debate on whether or not it was possible or even done prehistorically.
historically groups of people did in fact use inferior technology alongside superior technology quite regularly, in fact it wasn't too unusual to discard a superior method for an inferior one due to custom or politics, and have it stick for centuries. that is my problem with fantasy, generally it discards fantastic elements from history, in turn adds in magic fireballs and lightning and expects a believable world to emerge with non magical people being dominant over demigod level people, without using decent tactics or methods. it is like every time one does try to achieve dominance there is a jar jar to win the battle against him/her/it.