Well... Why is a Kink popular?
I think, if you are questioning one Kink, you are questioning all Kinks.
I (for example) can not understand Kinks like "Scat", "Feets" etc. These Kinks exist... I tolerate them, because they don't harm anyone. But I don't need to participate in something like that.
You can ask the exact same Question about all Kinks and you will always get different Answers.
What are People getting, when they play with poop?
Ok... for real, I would never ask this Question to become an Answer, because... well... It's an Answer to a Question I dont want to know.

Everyone has a kink that is incomprehensible to others, but what kinks, in turn, do "the others" have? Maybe Kinks that not everyone understands?
"Incest = Illicit" is a little bit too simple. In parts of the World, it's not uncommon.
But in that Parts, where it's "illicit" I think it's the Taboo thing, that appeals many interested people. The forbidden Fruit, that can taste so sweet.
Just my 2 cents...