[pinkpanther]

If the story is true and in circulation, why didn't the author of Mark, or Matthew, or Luke, include it in their books?

 It is also found in some older manuscripts of the Gospel of Luke..Luke's Gospel includes a story about a "sinful woman" who is forgiven by Jesus (Luke 7:36-50)

While not explicitly mentioning adultery, the woman's "sin" and her subsequent forgiveness are implicitly linked to issues of sin and redemption, which can be understood as including sexual sin, including adultery.