Thats actually a pretty good question. I believe these themes aren't popular because there isn't that big an audience for them. Also, it's a touchy subject when you're dealing with incest because then it borders on the child abuse territory.

A while back there was a small company making incest related porn, but they shrunk to mail-order only when someone disclosed the "family's" weren't at all related and the "kids" weren't at all kids.
I remember they had a line of videos called 'What's a Mom to Do' which was more like the videos on Myfriendshotmom.com except, the son would come home and be like "dude, why are you fscking my mom?" and the friend would go "coz she's hot dude!" and the kid would go, "you got that right. Mind if I join in?"- so as you can see it's not exactly what incest folks want.

The audience for incest are very senstive when it comes to such matters. Either give them a real family doing the "do" or give them stories. For some reason, they love stories. And you know how porn is nowadays- undress, boink and go! No time for big productions with lines etc

To be perfectly honest, the .gov is the real mongol. Why it wasn't too long ago that a couple of Virginia was arrested for making and distributing consensual scat-porn from their home. You may say- well certainly there is more to the story I'm not telling you, but the truth is that all they did was shit on one another and sell it to the good citizens of planet earth. So imagine if this couple decided to boink their kids!!!

You touched on the 80s as being a period consumed with incest related porn and this is true for only 1 reason: the popularity of video tape. Mindset is: if you can record it, then it's good to go!
A couple of scotts got arrested for boinking some kids and filming the entire thing. Turns out they knew balls about the law about boinking kids and recording it.

If this is indeed your prefered medium of perversion, I would suggest you check illustrations that deal with this subject because I'm not sure your going to be legally allowed to own it anywhere (I could be mistaken). Either that, or move to the deep Southern States of North Amerikkka where not only can you indulge yourself in this perversion, but you can live it too.

Good journey to you sir.