I've been thinking about this for a while (couple years). It seems to me, that besides the graphics being better than the average game using unmodified free daz assets or honey select, the thing that really made this game unique was those damn hidden cards, some of them only 1x2 mm in size (on my screen, and it's a frigging big 34" 5k).
The story, while interesting in areas, was pretty much meh compared to some other writers. Not as bad as some that should never have even bothered with English. Not as good as others.
tl;dr: it was the puzzle and collecting aspects of finding the hidden cards that made this unique.
(Oh, and even though the whole "talking about other games" in this thread has bounced back & forth between tolerated & deleted/locked... are there any others with that hide-and-go-seek aspect to it? Doubt it, since I feel it was in this soley "b/c I'm the greatest dev ever and give no hints b/c fuck you if you can't find 'em" type of "dev".
Sorry, but that's quite the bs, this game certainly didn't draw in thousand of patreons because of collectible cards, even though it was a nice gimmick. A lot has to do with when the game appeared:
- graphics for that time were quite decent
- web wasn't flooded with incest games back then. It was still a new and exciting kink. At that time really only ICCREATIONS and Inceton did incest games which could measure up.
- the setting actually is quite well made. It's lighthearted and allows for easy immersion in every boy's wet dream of having a family harem. Of course it had its technical flaws and unrealistic moments (like mother giving son a blowjob in the middle of a restaurant) but overall it was much easier to identify with this situation than in all of these overcomplicated tryhard blockbuster stories you find on the website. Unless dev really is a good storyteller (as in desert stalker for example) most of these stories are just plain boring and/or really way too complicated to really fall in love with. So in a sense it was good Icstor sticked with this really simple plot. Not because it was genius but simply because he avoided many mistakes with this.
- no NTR (thank god)
- the user interface back at the time was quite user friendly and had a "fresh look"
- in many scenes the music matched the theme quite well and created a nice atmosphere (Photoshooting older sister for example). It felt good and refreshing.
I am no Icstor fanboy trying to justify whatsover the scam he is pulling off, nor did I ever pay a single dime to him. I was just stating a few reasons why "I" think it ever became so popular.
Of course by nowadays standards many of the points mentioned above do not any longer apply, although thinking about it, there really are not too many "lighthearted incest games" currently being produced, considering that iccreations, lewdlab and SuperAlex basically have halted "production" or lowered their release schedule to maybe an update per year. On the top of my head only really only the NTL game comes to mind, who produces good family content on a regular basis and with a somewhat higher standard on appearance.