Taboos are hot, and incest is one of the ultimate taboos, and therefor ultimately hot. It's unlikely that it transfers into the real world. The number of incest porn enjoyers that actually lust after their relatives is rather small I imagine.
Forbidden fruit is often enticing. If it wasnt forbidden I doubt so many would like it. I think theres a big difference in fiction vs reality though, just because someone may enjoy the fantasy doesnt at all mean theyd want to recreate it in the real world.
Incest has that naughty taboo attached to it.
Why it's becoming a bit more mainstream? not really sure tbh
If I had to guess maybe society is becoming more lax with the taboo (knowing its not real aka step-bro, step-dad, etc.)