another reason the game did not sell well is that he did not even promote it . a quick look at his patreon page shows no existence of the port and the game has not leaked here so most people over here have no idea of its existence here either.
Promotion is done by the publisher mostly. Denpasoft just doesn't have the reach which Kagura Games has or anywhere near it if we're going to be honest. If you compare social medias, discord, sales on games and such, you will see a clear cut difference between both.
Still, using "estimations" in these types of games is a poor way of judging seeing how most people won't re-review a game they "already reviewed" and the game is no longer played through Steam once you apply the R18 patch meaning that the way those engines calculate purchases is number of players and reviews.
They also sell in other platforms that aren't Steam.