Because there was a finite period of time in which the Apostles were still alive and transmitting the Gospel orally?
That there was a finite period of time in which the Apostles were still alive and teaching orally and said teachings were being inscripturated does not prove the Roman concept of Tradition, whatever that ambiguous and amorphous concept may actually currently mean.
Why do you simply assume that the content of the unwritten traditions referred to by Paul is different from that which was written down? You never inform us of any of its content. What, exactly, is the information content of this unwritten Tradition referred to by Paul? Please produce the content. If you cannot produce any of it then how in the world could you possibly you know that the information content of the two is different?
To conclude from the mere fact that at one time the Gospel was given orally it necessarily follows that the unwritten content is therefore different, and that God thereby intended an additional binding rule of faith other than Scripture via Tradition, and not only that, but an extra-scriptural rule of faith to be administered exclusively by Rome, requires such gigantic, unfounded leaps of logic that it defies description.
Let me try anyway. It's like watching the current Supreme Court "interpret" the Constitution.
Let's keep it simple. The verse prior to the one you quoted from 2 Thessalonians 2 speaks of the Gospel. If you have any of the content of the oral teaching of the Apostles regarding the Gospel that is different that what is preserved in Scripture, let's see it.
Cordially,
No where does Christ say folks will write a New Testament, there will be an official canon somehow and this and only this is what you'll use to know my teaching.
He does, however, establish His Church, give it authority, select his messengers and, as we see in scripture, they select their replacements, and so on.
This is the way Paul taught, the Apostles and their successors teach. This is biblical where sola scriptura is not.
The teaching and transmission of the correct principles of Christian faith is through the Church, both orally and written - with the written interpreted by the Church - and by other means.
Not everything religious is capable of reducing to words, and not all teaching can be given and graded by words. This is not math. It involves people transmitting knowledge on all levels to other people.
If you wish some examples of words transmitted through the Church, a good source is St. John Damascene who produced a master work of compiling Orthodox teaching of the early Church. You can read this online: An Exact Exposition of the Orthodox Faith by St John Damascene.