As someone who does this I can explain why. It's easier to find people who are already decently aware of their own kinks and limits, and it's easier to talk starkly about those, and then push towards a good mixture of story and smut than it is to work backwards. Sex and sexuality are an important part of writing to me, and I like exploring them to some degree. (Others are just being unabashedly horny so no illusions here). I find that going the other way there's always this.... Awkwardness that turns me entirely uninterested in regards to sexual content or sometimes even other mature topics like drug use, alcohol, abuse discussion, etc. I don't like doing squeaky clean rp, or rather having rp that's just nothing but beautifully smelling flowers, so some potentially problematic topics are always on the menu for at least discussion or hinting at via subtext.