There are different angles from which one can answer your question. Given that most of the replies before mine mainly deal with credit card companies' policies, I'd like to offer an explanation from a legal standpoint. I'll try to keep it brief, so as to not bore you with all the "legalese."
As you initially stated, let's focus on American laws.
Generally, porn is protected "free speech" under the First Amendment to the U.S. Constitution.
Of course, there are certain forms of "speech" that are not protected under the 1st Am., such as speech that is likely to incite criminal activity, libel, "true threats," and
obscenity.
You'll notice that I put "obscenity" in bold because that's where our focus lies in this discussion.
Now, what qualifies as "obscene?"
Because we tend to reach for the top shelf in our day-to-day conversations, words such as these lose their true meaning.
Generally, something is obscene if it is "distasteful, offensive, and/or sexually explicit."
Next, what qualifies as "distasteful, offensive, and/or sexually explicit?"
Well, for that we must turn to the 3-part test that was set forth in
Miller v. California.
Under that test, visual works, taken as a whole, are considered obscene when they:
1) appeal to the prurient interest in sex;
2) portray sex acts in a patently offensive way; and
3) lack any serious literary, artistic, political, or scientific value.
Keep in mind that all three steps must be satisfied in order for something to be deemed obscene.
However, your question primarily focuses on the first step; prurient interests in sex.
Categories that usually fall under prurient interests are those that are shameful or morbid.
This is where incest comes in. I'm sure that we can all agree that, in real life, mothers having sex with their sons or fathers having sex with their daughters, etc., would be pretty darn shameful. How would you feel if you found out that your dad was having sex with your sister? You'd be ashamed and/or angry, right?
How would your neighbors/community see you and your family if they found out that you were having sex with your mother? They'd be disgusted, no? The whole affair would be morbid.
Yes, American society is hypocritical because we portray ourselves in a certain way to the outside world, but behind closed doors, we allow our true selves to come out, which includes our "primal" instincts; i.e., fucking anything that moves, even if it includes our own family members.
So while "step-cest" is the primary way in which big companies like Brazzers and Naughty America get around the "obscenity" of incest, that doesn't mean that those same companies bring the genre to the forefront. If you look closely, they market their scenes as pure sex. If you want to look at step-cest, you have to do some digging around on their site. The stuff is there, but they do a good job of mixing it into the sea of other scenes they make so as to not attract the ire of good 'ol American family values.
As such, this is why incest porn is not MARKETED as much as it is watched. In other words, incest is very mainstream, but we just don't like to admit it.
