Well, Ken Bailey's landmark book Jesus Through Middle Eastern Eyes offers a fascinating, culturally plausible theory why the pericope does not appear in some manuscripts:
For centuries traditional Middle Eastern culture has understood the honor of the family to be attached to the sexual behavior of its women. Thereby in conservative traditional village life, women who violate the sexual code are sometimes killed by their families.
Added to this is the fact that in the days of hand-copied manuscripts, the person who wanted a copy of anything usually acquired it by hiring a copyist. This was a private business arrangement. Since printing began, official committees of churches have determined the text of any Bible selected for publication. But in the early centuries of the life of the church it would have been very easy for the head of a household to take a copy of the Gospel of John to a professional copyist and say
"I want a copy of this document. Please leave out the story of this adulterous woman. I don’t want my daughters committing adultery and telling me, ‘Jesus forgave this woman and therefore you should forgive me!’"
The copyist would naturally oblige his customer. Other Christians were brave enough to preserve the story even though it violated deeply rooted cultural attitudes. The end result is that this story appears in some ancient texts and is missing from others. If this view is accepted, or if one considers it an agrapha, the story is authentic to Jesus. Raymond Brown writes, “There is nothing in the story itself or its language that would forbid us to think of it as an early story concerning Jesus.” Brown also notes, “Its succinct expression of the mercy of Jesus is as delicate as anything in Luke; its portrayal of Jesus as the serene judge has all the majesty that we would expect of John." With Metzger and Brown, I am convinced that it is a historical account.