Incest has existed throughout human history; there have been numerous cases in society. It appears as early as the Bible—Adam and Eve, Cain and Abel, Abraham and Sarah (children of the same father but different mothers), and Lot’s daughters, who got their father drunk to conceive children by him and preserve the lineage after the destruction of Sodom and Gomorrah.
Most famous historical case
It only became a sin after the introduction of the Law of Moses (Leviticus and Deuteronomy).
When the nation of Israel was formed, God established a strict moral and civil code. Chapters 18 and 20 of the Book of Leviticus detail the absolute prohibition of sexual relations and marriages between close blood relatives and relatives by marriage. Judaism—as well as religions based on Judaism, such as Christianity and Islam and their offshoots—condemns incest as a sin.
From Greek history, we have the famous Ptolemaic Dynasty, which ruled Egypt after the Greek conquest; they believed in maintaining a strong lineage through marriage. Ptolemy II Philadelphus married his paternal half-sister Arsinoe I and later his sister Arsinoe II; Ptolemy III married his sister Berenice II; Ptolemy IV married his cousin Arsinoe III; and so it continued, leading up to the most famous figure of all, Cleopatra VII.
However, in medieval Europe, there were also many cases of incestuous marriage. It is worth noting that marriage between cousins was not considered incest. Often, the bride's origins were omitted or erased from history; there are records of adoptions among noble families where, for a period before the wedding, a woman would be adopted by another family only to return later through marriage.
As a side note: incest is not a crime in most parts of the world provided both parties are of legal age and mentally competent, though such marriages remain prohibited due to family legal structures.
Sex is love and affection

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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.)
In my view, there is a stronger crackdown now; payment processors, religious groups, and feminist organizations are heavily targeting this subject. Production companies have switched to themes like "step-brother" and "step-dad" to avoid scrutiny. Family Strokes, for instance, has removed many films from its site, though many can still be found elsewhere online; previously, the content involved mother-son and father-daughter dynamics—examples include the *Taboo* series (1980) and *Nome do Pai* (1978), among many others from the 70s and 80s.
Games featuring incest were banned from Steam last year due to pressure from payment processors, just as Patreon took similar action in 2017; the same applies to other video distribution platforms.