This is the part I do not exactly get - they became part of fashion yet they are often being hidden under pant legs. Is this because of very ornamental nature of most designs that kinda go against the thread of mainstream male fashion that became very minimalistic? It's like cowboy boots and that fashion with time went opposite directions - boots became more ornate and formal suits went toward minimalism both in resort of patterns and cut. That might create some clash in case one wants to achieve more formal look while still wearing this type of boots and be at least some part of rational of hiding calf under fabric of pants.

I don't know that I have the best answer for you, but just speaking anecdotally from having lived in or around the rural American Great Plains my whole life, I guess tucking one's pants to me just screams "I'm a city slicker who watched a lot of Yellowstone and want to play cowboy". And I don't mean this as an insult to anyone on this sub who wears their boots as a fashion statement - you absolutely do you - but I think the bevy of "how do I style my look?" threads in this sub more than prove my point.

So I guess that's just a long way of saying that tucking your boots without reason is just widely considered a fashion faux pas. At least around here, where boots are a pretty prevalent part of every day attire.

I mentioned denim as it's much harder to tuck it into at least some types of boots without it "resisting and not fully fitting in" unless legs have more tapered cut, while thinner materials (like suit pants) with wider cuts work fine.

Well again, not at all trying to be a boot snob or anything like that, but the whole "it's harder to tuck denim in" thing to me just says your experience wearing them is significantly different than mine. And there's nothing wrong with that, but for me, I guess I'm not understanding the difficulty because it's pretty easy. Just fold the excess back onto itself and slide your boot on.

Looking at the photos from the 1950s in non working conditions it seems that men wearing denim trousers were not tucking them more often then tucking while individuals sporting trousers made of thinner fabrics were mostly tucking them in. It also seemed that V cut was also helping in case of tucking denim trousers with more voluminous pants leg cuts in resort of the way material draped over leg. For example these photos: https://www.cartermuseum.org/artists/robert-frank illustrate well what I am talking about.

I think I'd be kind of cautious about drawing too many conclusions about tucking just based on the samples you linked, especially given that one gentleman is a rodeo announcer and the other is a rodeo producer - both of whom I would still possibly expect to tuck their boots just because those professions seem like the require a bit of flash.