Its difficult to mix historical armor qualities with the D&D AC system. Hard leather armor is much more protective than its AC would lead you to believe. Likewise brigandine of good construction can afford better protection than mail on average.
These issues are inevitable when you try to add simulationist elements to a system rooted in abstraction.
Yeah, and I think supply and demand, as well as available wealth, is an even bigger impediment to matching the simulation with the abstraction. For example, you've got a bunch of mercenaries/landless "knights"/town levies running around that would be quite happy to upgrade whatever armor they have to something they think is better, but they can't afford it. To model this, you have to go way beyond what works in combat into the whole economic system of the world being simulated. But if you want "heroic fantasy", the disadvantages of going that route can quickly overwhelm any benefits. (If you want gritty, realistic fantasy, it might be another story.) Plus, there are only so many skilled craftsman engaged in making the best armor. Not everyone can have it.
There are also upkeep issues to some armor that aren't typically modeled. Leather is pretty good protection, but can deteriorate rather rapidly if not carefully maintained (oiled, and so forth). Chain is, against many weapons, almost as good as Plate for a faction of the cost, but takes a disproportionate amount of constant work to avoid rusting. (I think those details are correct. But I'm not an expert. Something like those issues are correct, even if those details are wrong.)
What might work in a more abstract, heroic fantasy style, is to somewhat abstract things like wealth and social station and upkeep, and then make these resources that a character has to invest in to get some of the better armor. If you invest enough in "landed knight", then you can afford plate, which is simply better than some of the cheaper armor. But then the guy wearing chain or heavy leather got to spend those resources learning something that gave him a different advantage. From a simulation perspective, that even make rough sense. That landed knight has responsibilities to go with that resource.
Of course, any such abstraction will have some edge cases that impinge on the simulation side. You can't get around that. But it might give you "good enough" for heroic fantasy, at a tiny percentage of the drawbacks of detailing everything.