Because there are few other places to find it.
If people want it, they will go where it is. If only a few places have it, those few places will have a large clientele.
Sounds like someone found the new search function.
High school and college stories can be found in far more places, and adult stories are all over the place. The more taboo categories are limited to fewer places, Nifty being one of the most popular and well-known.
And, stories about adult relationships are not only available in far more places, but they are often of much higher quality content as well as presentation at other places.
In doing the First Time Tales, I've found that the largest percentage of those whose first sexual encounters were before 18 were with best friends, but the next largest group is family members, mostly cousins then brothers. If all the stories I've been sent are actually true, that is. I wonder about quite a few of them.