Why do so many people write/enjoy familial stories?

I find it quite curious, in the main ballbusting subreddit there's 0 hints at this yet basically all top stories here are some variation of my step-sister, sometimes literal mom stories. I am not kink shaming or anything, I just can't make the connection and I find it very curious to see such an overlap/popularity on incestual content.

Why do you think the step-someone/sister found my balls stories in particular gets so many people off?

edit: just to be clear, personal preference aside I am quite interested in the topic of how people's relationships with kinks are, and I want to occasionally write so it's insightful in that way too. I don't mean to be the kink police or any such.