There will always be varying reasons why people like certain themes and/or fetishes... Sure, there is a very minute number of folks who may have actually had some real world thoughts and/or feelings when it comes to stuff like incest... But I think there are 2 primary reasons this particular theme seems to be "popular"...
1st primary reason is because the idea and feeling of reading/playing something so forbidden by society, gives this added sense of doing something even more naughty, to an already naughty theme of being an erotic/adult themed visual novel and/or game... In the same sense that some folks have an increased sexual pleasure when doing naughty things in public settings (exhibitionism)… It gives an extra umphh, to the already sexual naughtiness of a situation, for some people...
2nd primary reason is because at one point, several years back, there was this key moment where just a couple VN/Games, that included incest as their story base, were crafted very well, pulling in a LOT of money... While at the same time, sites like Patreon were beginning to ramp up in usage by this niche (erotic/adult visual novels and games) of the porn industry... So you add in this new donation system that was easy to use and was subscription only... Then combine it with a few VN/Games making lots of money using this same system for their donations... And suddenly we began to get tons more developers coming out of the woodwork, using the same themes to try and get a piece of the pie... In essence. either riding on the coat tails of those big hitters, and/or copying some the same basic concepts to include incest... What we ended up getting was a shit storm of incest themed VN/Games, of varying quality and design, flooding the market for several years... With such a flood of the market focusing on mostly one over all theme, it was like a snow ball effect... Artificially creating this idea that Incest must be the most popular of themes, thereby you get even more developers wanting to use this same theme to perhaps get more paying fans, all because they used incest, so they could maybe make the same kind of money as those big hitters that sparked off this trend...
Of course, the market was also flooded with low to mediocre quality VN/Games, due to the influx of so many newer developers, with little to no experience with creating these things, let alone including Incest as the over theme... Many eventually became abandoned, a few did very well and eventually finished, while even more are still lingering in development limbo (still in development for what seems like forever)…
But I do think that we've already gone past the point, probably about a year or so ago, where this particular theme of incest has began to take it's tole... Mostly because too much of just one thing, has this tendency to lose it's luster when it is used too often and in the same pattern of use by too many developers... What keeps a market stimulated is variety... Too much of one thing or the other just stagnates the market... Sure, you will have some who will always want more and more of the same old thing... But the market, as a whole, thrives on variety and newness, not just more of the same thing, practically always done in the same way... That is why I think that we are finally seeing more developers shy away from this particular theme, if not shying away from how it is typically patterned for use... And we are seeing a rise in other themes, and/or content outside of this one particular theme of incest... Not that it's completely dying, but instead, the market is trying to even out the playing field a bit... Mostly in light of rules changes by sites such as Patreon and others, along with the over all playerbase/readership becoming a bit bored of seeing the same old thing all the time... Variety is the spice of life...
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