Why do Abyss monsters drop no loot?

No matter how high i roll +currency on my map or which scarabs i use i cant make all the abyss rares drop anything significant. Which game mechanic decides which monsters drop a lot of loot and which dont? Because other league mechanic monsters like breach clearly drop a ton of loot. But essence is another mechanic where i notice the monsters never want to drop anything valuable. I rememeber there was "hidden quant" but i never understood this and how players figure this out, also afaik it was removed in kalandra league and from chris post he explained all hidden quant was moved into archnemesis. more monster mods = more quant. Doesnt seem to be true anymore, with nemesis scarab being less than 1c (and from my testing anyways). So i didnt even understood the first system used and it seems to have changed multiple times now. If anyone can help me catch up with that please.

Unsure, I don't run much abyss. However there has been a lot of talk in the community about how ground loot has change over the past few years of seasons, and apparently the Devs found a mechanic that was inflating drops, relating to league mechanics that spawn. They removed it some leagues back apparrrently, which I think is what you are refferencing. Someone made a video about it going through the leagues and the changes that were made, you might be interested in it or have seen it. It may relate to what you are asking about, over a period of some time. I don't have a link, but I think it is called the Kaladrafication of POE.

This league there was a lot of changes made, in the direction of buffing ground loot. Not sure exactly how that ties in with what you are trying to do. Personally I've never know these underline systems, they mostly related to SCTrade, maximizing currency per hour. There has been a lot of reliance on having the right combination of scarabs lately. I'm not sure what are current good farming strategies, so someone else might need to help you with that one. I don't remember abyss being good for currency for some time, but there's a lot I don't know about, particularly with currency farming, there probably are good strategies available.

I agree with you that breach does seem to be dropping a lot of loot currently. Part of that, I think, is about it being the most recent mechanic. The recent league is often strong unless it got heavy nerfs, because they don't usually run a league where the league mechanic is underpowered. I think you what you are suggesting is that if you are killing a similar amount of rare mobs in each mechanic, they should be dropping a relatively similar amount of loot. I basically agree. It's obviously a lot more complicated than that, across all the different league and reward types, and how they can be modified and multiplied. I think most the changes they have made this league were good ones. Removing past modifiers that were doing hidden loot buffs, might not be the best for the overall loot levels dropping, but It might help with the balancing of the game in general, and allow them to add buffs in the future, that aren't disproportionally increasing certain sections of the game that have these hidden buff unevenly and making certain strategies too powerful, and invalidating others.

In saying that, a lot of players do enjoy when the game is unbalanced, so they can find and use that strategy. I think within reason the mechanics don't need to be completely balanced, especially if they are providing different things. My main problem with abyss is that I find it annoying chasing the pathing, which is unpredictable and often makes you backtrack. If you are juicing up the maps it can be good, but otherwise it can interrupt the flow of going through them. I think at times in the past, it has been good for xp, in combination with other things such as beyond. I'm not sure how it sits with that currently. If it is not so good for that compared with other mechanics currently, perhaps it does need a buff to currency or the interesting items that it drops. It could be that they are looking to rework it at some point, as I think it is one of the older mechanics now and it is one of the first things that was added in POE2.