It's that the majority of imageboard users are straight men and a lot of the more prominent guro/scat artists are straight men--or else pretending to be. Frankly, the internet is not a kind place for women or gay men, particularly not the chans. (I mean, using "fag" as a synonym for "person" is pretty offputting.) Female hardcore artists are much more likely to be shamed for it (and definitely more likely to face negative consequences if anyone in RL finds out), and women are more likely to avoid imageboards out of fear (or just not wanting to see so many lolis). So it's kind of self-perpetuating. If they have kids, they're less likely to find time to draw, or even write--but writing is easier to do in bits and pieces than art. If they're actually old enough to be any good at what they do, they're way more likely to have been discouraged from using computers growing up--or simply not encouraged; it's only in about the last decade that computers stopped being major pains in the ass or that having more than one per family was common--and so more likely to not know how to use digital art programs. Demographically, women and gay men are less likely to be able to afford either traditional art supplies or a good art tablet. (Say what you will about the "real" causes behind the pay gap; an "average net worth" gap certainly does exist. And while it's possible to make decent art with a mouse and free programs--I've done it, it's on my f-list profile, same name as here--it takes twice as long and it's really frustrating.) And then, if they do draw guys, they get a lot of being ignored or "ew no one wants that gay shit."

So that's why there's relatively little male guro *art* on imageboards and Pixiv; it's higher risk and a higher time and (proportional) money commitment for women and gay men to do it, and then the people who collect it into imageboards and themed threads tend to pass over male art. If you go to places where artists themselves post, such as DeviantArt and Pixiv, you will find a lot more male art, though still a lot less than female art.

Meanwhile, writing stories is accessible to anyone with Microsoft Word and a computer capable of running it (which is literally anything made in the last 15 years--compare that to the system requirements for making digital art without prohibitive lag) and a high school degree. It's easy to do even when you only have a few minutes at a time to work on things, and it doesn't require specialized fine-motor skills beyond what most US adults already have. So you see a lot more diversity in stories, both in subjects and authors.