Because they wanted to make sure there were no authoritative writings around that conflicted with anything written in the Christian Bible, and hence could possibly undermine the authority of the church. The modern bible is the product of the Council of Nicea, which convened in 325 AD with the blessings of the first Christian Roman emperor, Constantine. In preparation for the Council to meet, the message was spread throughout the entire known world the year before to assemble and bring every piece of major literature associated with Jesus and Christianity to the Council, so it could be considered. Sounds promising, right? When the Council convened, there were 52 submitted volumes of work detailing everything about Christianity and the life of Jesus. The Council only approved 4 of the 52, and completely threw out the other 48, because they didn't fit the narrative that the church wanted to tell. The only 4 approved were the gospels of Matthew, Mark, Luke and John - and everything else was ignored. After the Council was over, all copies of these other 48 book were ordered to be destroyed. These 48 ignored books became the first forbidden books. As the centuries passed, other books were added to the list that various clerics in power deemed embarassing or contradictory or they simply didn't like for whatever reason, and the list grew.