Part of it may be that superior knowledge was highly secretive and well guarded.
That's why I'm saying it's the same as today. There is no good reason a peasant would be unable to read, except that nobody taught them, which should not take too much time for alphabetics, or that it was written in a foreign, holy language.
Heck, Runes were considered to have magical power and ritual incantations were highly formularized--as they are today: a) e.g. in fashion brands, b) e.g. in law code to the extent that it requires professional translators
Or closer to the topic, take maths, which has a highly formularized, international, often ambiguous and domain specific writing system. Indeed, it's also a good example of a science where instruction is crucial, and most information is left out in writing because the reader is expected to have it all in working memory.
Even better analogy, computing machines: Not only do write with them, but in a sense we encode information in the, well, object, just like a sine and cosine wave diagram (or animation) encodes the motion of a radius in the circle. Only the keenest reverse engineers are able to read out the fundamental principles of its working. But, for analogy, CISC was found unwieldy so the trend is going back to RISC and doing hyperscalarity via networking, to solve heat problems.
Anyhow,domain knowledge implies today as it did then where to get stuff, not just how to use it. Specifically for the Iron age this means knowledge of iron mines, geography and geology. Maths and other structural sciences are only peripheral.
Given that early Iron was meteoric, one can kind of see how a connection to the skies and gods could be drawn.
And we have to wonder about fate, too. Some resources simply deplete. It's not that the knowledge is lost, but its application. Calenders have been further developed, certainly. Currently we predict the future on the end of global climate. Yeah that's sad, but it puts the little bit of heat that I put out as background noise into perspective.