Maybe mostly, because of this idea that the Bible, which includes the Tanakh, and the letters known as the NT, as complete and the infallible, God given word. Most westernized denomination, including the RCC, have forced everyone to believe that the OT and the NT must harmonize, even if the representation of God in the O.T. is not the representation of the a crucified God in the NT.
Go to any church in the U.S. today that is RCC or protestant and claim that the OT is not needed, they would deem you a heretic, tar and feather you and if legal, some might have burned you at the stake medieval style.
The reality is this, and I have pointed this out before in this forum. There is NO edict from God, Jesus, or even within the Bible itself that Christians need to have or will be given a 'Bible' or the need to create a canon of letters that include the O.T. and call it the Word of God.
The question that should be asked is this "What did the first century Christians ever do without a Bible", keeping in mind they were the closest to Jesus and his teachings and the gospel (Don't conflate gospel for the Bible as gospel means 'good news').