Two reasons:
1. God promised to preserve His Word.
2. God didn't promise to preserve 'apostolic tradition'.
Those two things aside, as honest God-fearing men the Apostles wouldn't have been double-minded writing one thing, preaching the opposite. Whatever their 'tradition', it would have to be aligned with their inspired writings.
Scripture testifies to the necessity of observing the apostolic oral tradition. You cannot reject the oral tradition without rejecting scripture. You have rejected scripture.
Where is this list of apostolic tradition? It doesn't exist, what passes for tradition is whatever the Catholic church says is tradition even when it contravenes God's Word. All information needed to believe Jesus is the Christ and to have life in His Name is contained in the Gospel of John. John 20:30-31. As the Catholic church has levied other requirements for salvation that don't exist in John's Gospel, it preaches another Gospel and is accursed. Making the rest of the argument irrelevant until the Catholic church repents and turns from its sin.
Jesus said that his words would not pass away. He said nothing about just the written words.
The apostles were the nearest witnesses to the words of Jesus. It was only they whose minds were opened to understand the scriptures.
Paul in Romans testifies that no one can preach unless he is sent. It doesn’t say self-appointed. It doesn’t say well-read. It says “sent”. Your teachers are not sent. You are not sent.
The Greek word for “sent” is “apostalosi”. All authentic teachers are sent, by successors of the apostles who were sent themselves. It’s the apostles and their successors who are the authentic custodians of Jesus’s teaching. If you don’t know the apostolic teaching, you don’t know Jesus.
“He who hears you hears me, and he who rejects you rejects me, and he who rejects me rejects him who sent me.”