Erotica demands that both the writer and reader has enough creativity to imagine the sex acts, and even characters, as well time to comprehend what's going on, and in this day and age, you can't really expect people to have a long attention span. I know I don't have one. Plus, we don't get traction. There are those who prefer reading porn instead of staring at it, sure, and it's much easier to write about it. But of course, some things are just not as attractive than seeing it - such as, say, a mating press or instant loss, where it's usually just two frames. Plus, it just looks like there's much more freedom in drawing than there is to writing. Of course you can decide on whether you want to write 1:10,000 word page or 10:1,000 word page... but audiences usually, once again, don't have the attention span to read all of that, and even if we write for ourselves, just how much entertained are we when we know what's going to happen, how to visualize that in our heads, and not get any feedback? That is all.
It depends on what aspects of porn you find arousing. Erotic drawings enable the creator to fine-tune the physical, visible aspects of a sexual scene - where every single body part of every single character is located, or exactly what each person looks like, down to the precise shape and structure of their nose. When someone ejaculates onto a character's face, the audience can see exactly where each individual drop of semen has landed, and how much of their face is covered, and their expression at the moment of impact. To convey equivalently detailed information in writing would take prose akin to grinding down your reader's mind with a mortar and pestle. Erotic stories enable the creator to express emotions, context, and the other four senses in ways that are difficult or impossible in purely visual media. It is much easier to capture a character's personality and thoughts, or relate the current sexual encounter to their views and prior experiences. When he drops his towel to reveal his throbbing erection, you may not see her eyes grow wide, but you feel her heart skip a beat, and you sense the growing heat between her legs. You might learn she's never seen such a girthy cock, or that she regrets putting on her expensive panties only to immediately get them soaked. I'm here because, of course, I prefer the latter medium. I don't particularly need to know the exact size and shape of some minor character's ballsack to enjoy what's happening. I can visualise the dress strap slipping down her shoulder just fine, but to know how excited her coworker is becoming as he gradually sees more and more skin, how amazed he is that this seemingly stern and serious woman is so flirty and playful outside of her shift, is invaluable to me. There are some ideas that are simply easier to express as a piece of visual art, but I generally prefer those that thrive on the written word. Those 'instant loss' comics aren't very interesting to me, because I want to know what happens between the two panels. I want to experience the sudden change in the situation and the conflicted feelings of the subject character. This is partly why women mostly prefer written erotica to visual porn, while men are the opposite (outside of CHYOA, which is something of an anomaly). Sure, a specific sex position is easier to illustrate with a drawing, but the complex sexual tension between a barista and a cafe regular is better conveyed through the written word. That is the freedom that the author has over the artist.
Plus I don't think you have any idea of how much more work would be to publish a short webcomic instead of a story on Chyoa.
For me for erotica to be a turn-on the context is the key. The written format allows the contextual fantasy to be developed in a far richer way than visual porn 90% of the time provides. Or in simpler terms live action porn generally doesn't have a storyline or character arcs. Conversly if I just want to see (imagine etc) attractive people have sex and I'm not in the mood for context then visual porn is simply better at doing that than 90% of even well written erotica. Porn comics are difficult because they can either get the best of both worlds or more often fall between them and not quite fulfil either desire. For me this is why I personally quite like adding in pictures and gifs to my chapters. They provide some raw visual stimulus to add to the written context. Just my 2 cents.
