An amateurish opinion:
You're missing the "hot" adjective, as in "hot cousin/sister/brother/father/mother/uncle/aunt/etc". How many people would be interested in incestuous activity with the ugly, fat, stinky, sweaty, creepy, crazy, asshole of a relative? Not enough to have a market.
He/she is hot, forbidden and approachable. For the ever increasing in number people with various degrees in social anxiety, not having to bear the public humiliation of a rejection from a complete stranger in a sea of complete strangers a.k.a. society (where they have majority vote), that relative that knows them since young is a beacon of hope, a person they can trust, an easily obtained safety net. Also, they are hot. Did i mention hot?
It's also a dare, a secret, a feat, an achievement, an ego trip. "I did what others didn't or couldn't! I reached a place few others have! I am better than my peers! I banged my ____(insert relative here)!"
Man or woman, the thought of being superior to others is something engraved within us and defines us as individuals. As long as there is an 'i' in 'team', there will be conflict and there will be a desire to surpass, in any way possible.
Back to it being taboo, rebels at heart are more likely to pursue the subject as well. Because it is against the law, frowned upon in society, despised by the majority, fought against by morals and ethics and generally unlikable, it almost becomes a compulsion to pursue it just to fulfill that need to piss people off.
Which brings us to this point: the power trip. We are animals, no doubt about it. We strive to better ourselves, but remnants of our ancient past remain to this day within our genes. Why am i bringing this up? Well, as you see in some of the recent incest games, the protagonist sees himself as used and abused by his relatives and looks to turn the tides, usually by blackmail, drugs or brute force.
This type of game resonates with the type of person who see themselves as overworked, exploited slaves under the iron rule of their relatives.
To take charge and assert themselves as the dominant pillar of the house is a way to relieve stress and avoid a mental breakdown due to the weakening of their ego.
There are reasons and causes for a favorable opinion towards incest, some are good, some are bad, some are needed, some are a pile of shit, but the desire in itself does not appear without a trigger, of any kind.
The best bet of combating this trend would be to reduce the effects of social awkwardness. If less people hide behind a keyboard and more meet others who can accept them and themselves as they are, the need for incest and a few other fetishes will lessen due to lack of time to worry about it.
A short quote from a novel i'm reading that i momentarily feel explains many... desires we humans have:
No one had ever managed to corrupt an angel. The same was true in my previous life. It was a myth. My heart jumped quickly. I wanted to try corrupting an angel.