Perhaps it is because the men who composed the books of the Bible were biased toward the written word.
There were astronomically more Christians who set a good example living Christian tradition than who wrote scripture. Based on ealgeone’s above logic, therefore perhaps the vast majority of Christians at the time of the writing were biased toward tradition.
One of the reasons we have the NT canon is so we can have an agreed upon set of writings with which to compare false teachings that arise.
We see examples of false teachings in Galatians and 1 John being addressed by Paul and John.
There are reasons the Protoevangilium of James, the Gospel of Peter, the Epistles of Clement, the Didache, 1 and 2 Clement, Shepherd of Hermas and others were not included.
I again refer you to F.F. Bruce's work, the Canon of Scripture, for more insight to the formation of the NT canon.
If one cannot read the NT and determine how one should conduct their life as a believer, then all of the tradition in the world isn't going to help them.