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.
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