Why was the Gospel of Thomas left out of the Bible?

Answer 1

It was never seriously considered. From its first notice by Hippolytus (ca. 220), it was generally considered by church leaders not to be an early authentic work, but rather a later Gnostic (hence heretical) text, both because it was found in use among Gnostic groups, and because it didn't conform to the church's developing orthodox doctrines. Measured as a gospel record, it attributes no miracles to Jesus, places no importance on his death, and doesn't mention the resurrection. From the prologue on, the focus is not on what Jesus did, but on what he said. Although many of the sayings have parallels in the canonical gospels, it was felt by church leaders that the rest of it, and its general tone, was unacceptable.