I have a pet hypothesis that, on average, those into step porn are more likely not have grown up with a sibling of the gender they fancy. I reckon those of us who did are more likely to repelled by anything alluding to incest.
People who grew up with ACTUAL hot step siblings notwithstanding.
I remember a post somewhere that suggested the wave of step-sibling porn is because it’s really effective short-hand. You get two characters that know each other and are close, but shouldn’t do anything because of all the fallout that would happen. It’s a quick way to recreate the childhood friend dynamic, but also add a strong taboo and explain why they always hang out in the same house. It also gives a reason why sex can be frequent, but never tie someone into a relationship.
Sure, there’s a lot of perverts in the world, but there’s also people who enjoy the dynamic of “we really shouldn’t… but…”
It’s also incredibly cheap to produce, requiring no unusual props or location shooting, and generally tolerable to those who aren’t interested in the kink, so it’s a relatively safe bet economically
And on top of that, unlike many other kinks, even people who don’t care about the step thing might still be interested.