Burgon, ibid p 147, states that the dislocation of John 7:53-8:11 (see notes under RSV and GN) is attributable to four cursives, 13, 69, 124, 346, all evidently from one ancient and corrupt copy.
Ruckman (2) p 134, cites in favour of the passage, the Didache (3rd century document of Apostolic Teachings), Apostolic Constitutions (4th century) and Eusebius (324 AD) citing Papias (150 AD) as recognizing the passage. The Montanists (2nd century) were also aware of the passage. Ruckman (31) p 333, also cites besides D, uncials M, S and Gamma from the 5th, 8th and 9th centuries in favour of this passage. Concerning authorship of the passage (see note under JB), Hills (38) p 130, states that "arguments from style are notoriously weak." Berry's Greek text supports this passage.
Many of the early manuscripts and papyri have it, the earliest church fathers quote it as scripture, and it is Johannine in style. Why then was it excised by some copyists? Probably because the early church got away from the radical message of grace very quickly. 
It is obvious to any born again Christian why Religionists would want to minimize or eliminate this scripture.
RELIGION SAYS: "Change, or I will condemn you." The fear and threat of God's condemnation is always potentially hanging over our heads in order to "keep us in line." This does not promote deep and lasting change. People who live under this tend to have a superficial and self-righteous kind of holiness.
GRACE SAYS: "I have forgiven and accepted you. Now respond to my love by allowing me to change your life." Loving gratitude is the most powerful motivator in the universe. We don't change in order to be accepted, but because we have been accepted. Real righteousness is practicing love toward God and others. Nothing motivates this kind of lifestyle like forgiveness received. This is why grace, properly understood and appropriated, produces a superior righteousness in the lifestyle of its recipient.