My theory:
In the early 90s, all the molds were for much bigger (9"+) boards with proportionate concave. When boards went from 9"+ to sub-8" over a very short period of time, the molds stayed the same but the boards were being cut narrower hence flatter concave and kicks. It also helps that a flatter board is more nimble and can be pretty helpful for the technical skating that was just becoming popular. Keep in mind that at the time, concave itself was a relatively new thing.
Once the pop was standardized, brands started experimenting with new molds (think late 90s/early 00s and everyone putting new molds and "tech" out there) and concave started to get steeper or vary more or whatever.
I read the initial post the other day, but had a conversation about it with others today (forgetting I had even read this) but related to mold sizes and concaves for different decks, eg making boards proportionate to the size, length and concave so they are all evenly balanced is a very important part of making a skateboard deck and something that should never be overlooked or taken for granted, specifically related the the Antihero eagle range which are all very well balanced and could very well be ten different molds right there, from 7.3 7.8 8.0 8.125 8.25 8.38 8.5 8.6 8.75 to 9 - pretty cool to see a 7.3 beside a 9 and I wish I had more pics when I had both at the same time.
Agreed that when things went from super wide (and very big, long, squared off boards) to fairly narrow (and very rounded, shorter wheelbase, smaller kicks) that same mold would barely have given those boards much concave across and much smaller kicks too.
Then because boards were smaller and flatter, plus people were doing a lot more heavy impact skating, they were breaking faster, so again the industry could not keep up so boards were not being made as well or given time to cure, which only caused more boards to break more easily, in a downward spiral for a few years until things finally caught up and levelled out.
The above sounds about right. I remember when I first bought a late 90’s Black Label and I thought the concave was crazy. I liked it though and was really into all those Giant Distribution era Black Label, New Deal, and Element decks.
Yeah some of those 2000 era decks were so steep, they would put FA / Hockey in the "mellow" basket, but I am glad it all evened out and has been much the same for almost the last twenty years now I think.
I can't remember who, but someone had said when they made them with more concave mid 90s finally, a lot of people didn't like them, so they went back to molds with even more flat for some boards too. When I stand on the only board I have from '92, there is more concave than another board I bought that was from '96 and it is so flat it almost doesn't have concave, but it is there, just slightly though.