It's not new, just something the porn industry didn't want to touch before. The erotica and hentai game industry was all over it by the late 1990s.
Anyway, skip to the bottom if you want to just get to my point, I wrote a lot.
And it's because it's a high demand keyword. While "lesbian" is number 1, it's the only thing relating to it - and it's perhaps the most heavily contested niche - while porn sites have only recently broken into the incest categories - which are #4, 5, and 6 - and their volumes are WAAAAY more than lesbians. MILF - for many people was more or less a euphemism for "step-mom" but the porn industry wasn't ready to go there until later.
Side note - I'm pleased to see massage porn get so high! Speaking of massages: if you want to get a woman in the mood, they are a good way to do it - even if your wife has a "headache" =) There are two tendons just below the base of the skull beside the spine; more good points around the ears - and it makes that "headache" problem go away in under 5 minutes. If by chance you have a time machine to go back to University, massages are around a 60% ticket. Also, watching massage porn with your other half is a good way to get her in the mood... and lesbian porn, this might be couples watching to get in the mood; in my experience, many straight women get off on that, and men, we're fine with it too =P - even if we can't place ourselves in the fantasy without a bit of imagination.
Back on topic. I think a lot of porn is fantasy, and people like to fit themselves into the place of someone doing something they want, but would never ever consider doing it in real life, even if given the opportunity.
My (mostly conjectured) hypothesis: A more monoculture society would have similar people as being a big part of their lives, and therefore might develop a subconscious attraction to related people - while it's a stereotype, there is something to it. In areas of the US where there are large populations of black people, interracial (specifically refers to black and white in porn terms) and ebony porn is HUGE - because people have a kink for seeing a black man and a white woman, or a white man and a black woman getting it on. So, in areas where the population is monocultural with stronger family bonds, incest porn will be a lot bigger.
It might also be a rising trend with the most recent generations - particularly younger millennials and generation Z. They are living with their families LOOOONG after we did in our time. I know several of us at the Shrine were out of our parents home by the age of 16, and most of the rest by 18 - but since the economic crash of 2008, a large percentage of people seem to be staying in their parents home. I suspect this cultural shift may have something to do with the change of people's sexual fantasies - although, again, I have little evidence other than comparisons with other categories.
So here're another couple of hypotheses. Take an age - let's say 28 years (people 18 in 2008) - and I will bet incest porn is MUCH more frequent among the 28 and under category than the 29 and older category. I'd also suspect that, regionally, in the 29 and above category, those who live in diverse multi-cultural regions will have next to no incest porn searches, while those who live in more monocultural regions have moderate to high results in their searches.
