I think step porn is so popular because it basically establishes a relationship beforehand, even though it is familial. This gives our guy or whatever proxy for the audience they use a preexisting relationship (creates a taboo and and a reason why they know each other), proximity and opportunities for interaction, no need to go to a bar or whatever (makes the script easy to explain why they are forced together in a situation.

I think it makes creating porn easier since it quickly explains why these two or more people are around each other in a lonely house or hotel room (ease of filming venue), gives them a preexisting relationship with maybe hidden desire (no wooing or any modern prerequisites before meeting a potential sexual partner), it’s a bit taboo but easily ignored by the majority of the audience, and finally an established easy premise to follow and write with a lot of variation to the formula.