Well, they're asking to be pirated.
For western market, they tried to put their games on steam, but they cut alot of content (turning KK into KKP) while they put AAA pricetag, so people pay more, for less content. Also they only list a very few of them, and not even include HS, which arguably their most popular release.
You can just torrent a repacked version with all of the content, dlcs, and essential mods with a simple click, free of charge without missing anything, so what's the point in buying?
They didnt utilize steam workshop at all, so, again more reason not to buy there. There are various discord community out there where people can just join and share/download any kind of content there.
In addition, even though H games has been around in steam for a while, it still haven't widely "accepted" yet. Alot of people still doesn't want others to find out that they're been purchasing and playing adult games on their account.
There's also the biggest problem.. Their game fucking SUCKS. KK/HS/HS2 overstay because of their character maker and Studio with incredible mods and community. It's literally GMOD with porn, or MMD for dummies, they're so beginner friendly that anybody can do it given enough patience and time. But regarding their gameplay of the game itself, it's just very bland. If you see their sales on DLsites and such, they got absolutely destroyed by smaller studios such as miconisomi. Simply because miconisomi makes an actual game, while Illusion has only been releasing "rehash" of a broken mess of a game without any new animation, scenes, and actual good gameplay.
They can either do the KISS thing, where they stay dormant and let the community handle the game, or make an actual, fresh new game with an actual gameplay. Instead all the did was waste budget by making garbage like RG over here with prettier graphics as its only selling point.
So yes, piracy took part of it, but mostly it's because of their own poor decision making.
It's pretty sad actually. They just recently announced their 30th anniv and the goods on their site, got a feeling that they doesn't even see their parent company pulling the plug until the last minute.