Because only scripture is inspired by God and if someone comes and puts in tradition that don't line up with scripture then someone somewhere goofed up and it isnt scripture.
So your saying that there is no where in Scripture where it explicitly says tradition must be backed up with Scripture?
Something else I don't think is recognized here is that the tradition in question pre-existed the New Covenant Scriptures which were a result of said tradition. Although all of the Gospel is written truth, all truth is not written, only a smidgen.
There are two modes for one stream of revelation:
- By "Letter", the written word, the Bible
- By "Oral Statement", handing on, paradosis, tradition
The community of believers, the Church, sought to know what God authoritatively revealed to humankind.
The Bible itself does not define what it includes; nor does it claim to contain all that God revealed.
Paul affirms that
some of what is handed on--the way Jews passed on revelation--was "by letter," in writing.
What is the standard--the canon--of the written Word of God? The Church recorded the history of the development of the canon of the Bible under the authority of the Holy Spirit.
A primary criterion of canonicity is inspiration, the divine influence on human writers such that God is said to be the author. And God's revelation, faithfully written, aptly expressed, was expressed with infallible truth.
We discover the meaning of the Bible--hermeneutics--by the literal sense of the words first, then the fuller sense, and then the typical sense of the words of Sacred Scripture.
1 Cor 2:9-10 But as it is written: "What eye has not seen, and ear has not heard, and what has not entered the human heart, what God has prepared for those who love him," this God has revealed to us through the Spirit.
Mt 16:17 Jesus said to him in reply, "Blessed are you, Simon son of Jonah. For flesh and blood has not revealed this to you, but my heavenly Father."
2 Pet 1:21 for no prophecy ever came through human will; but rather human beings moved by the holy Spirit spoke under the influence of God.
Rom 1:19 For what can be known about God is evident to them, because God made it evident to them.
Jesus has told us that he has not revealed all truths to us.
Jn 16:12-13 I have much more to tell you, but you cannot bear it now. But when he comes, the Spirit of truth, he will guide you to all truth.
Jesus then told us how he was planning to assist us in knowing other truths.
Jn 14:16-17 And I will ask the Father, and he will give you another Advocate to be with you always, the Spirit of truth, which the world cannot accept, because it neither sees nor knows it. But you know it, because it remains with you, and will be in you.
Jn 15:26 When the Advocate comes whom I will send you from the Father, the Spirit of truth that proceeds from the Father, he will testify to me.
This all means sister that there is truth outside of the written word. The communion of the saints though implicitly outlined in Holy and Sacred Scripture is one of these truths explicitly outlined in oral tradition and the paradosis of the followers of the way.
Before the canon of Holy Scripture we see writings like these that further substantiate the claim:
"[T]hat it is neither possible for us ever to forsake Christ, who suffered for the salvation of such as shall be saved throughout the whole world (the blameless one for sinners), nor to worship any other. For Him indeed, as being the Son of God, we adore; but the martyrs, as disciples and followers of the Lord, we worthily love on account of their extraordinary affection towards their own King and Master, of whom may we also be made companions and fellow disciples! The centurion then, seeing the strife excited by the Jews, placed the body in the midst of the fire, and consumed it. Accordingly, we afterwards took up his bones, as being more precious than the most exquisite jewels, and more purified than gold, and deposited them in a fitting place, whither, being gathered together, as opportunity is allowed us, with joy and rejoicing, the Lord shall grant us to celebrate the anniversary of his martyrdom, both in memory of those who have already finished their course, and for the exercising and preparation of those yet to walk in their steps."
Martyrdom of Polycarp 17,18 (A.D. 157).
"[Appealing to the three companions of Daniel] Think of me, I beseech you, so that I may achieve with you the same fate of martyrdom."
Hippolytus of Rome, On Daniel, 11:30 (A.D. 204).
"As often as the anniversary comes round, we make offerings for the dead as birthday honours."
Tertullian, The Crown, 3 (A.D. 211).
"Nor is that kind of title to glories in the case of Celerinus, our beloved, an unfamiliar and novel thing. He is advancing in the footsteps of his kindred; he rivals his parents and relations in equal honours of divine condescension. His grandmother, Celerina, was some time since crowned with martyrdom. Moreover, his paternal and maternal uncles, Laurentius and Egnatius, who themselves also were once warring in the camps of the world, but were true and spiritual soldiers of God, casting down the devil by the confession of Christ, merited palms and crowns from the Lord by their illustrious passion. We always offer sacrifices for them, as you remember, as often as we celebrate the passions and days of the martyrs in the annual commemoration. Nor could he, therefore, be degenerate and inferior whom this family dignity and a generous nobility provoked, by domestic examples of virtue and faith. But if in a worldly family it is a matter of heraldry and of praise to be a patrician, of bow much greater praise and honour is it to become of noble rank in the celestial heraldry! I cannot tell whom I should call more blessed,--whether those ancestors, for a posterity so illustrious, or him, for an origin so glorious. So equally between them does the divine condescension flow, and pass to and fro, that, just as the dignity of their offspring brightens their crown, so the sublimity of his ancestry illuminates his glory."
Cyprian, To Clergy and People, Epistle 33(39):3 (A.D. 250).
"I am also of opinion that there were many persons of the same name with John the apostle, who by their love for him, and their admiration and emulation of him, and their desire to be loved by the Lord as he was loved, were induced to embrace also the same designation, just as we find many of the children of the faithful called by the names of Paul and Peter."
Dionysius of Alexandria, Books of Promises, 5 (A.D. 257).
"
Then we commemorate also those who have fallen asleep before us, first Patriarchs, Prophets, Apostles, Martyrs, that at their prayers and intercessions God would receive our petition. Then on behalf also of the Holy Fathers and Bishops who have fallen asleep before us, and in a word of all who in past years have fallen asleep among us, believing that it will be a very great benefit to the souls, for whom the supplication is put up, while that holy and most awful sacrifice is set forth."
Cyril of Jerusalem, Catechetical Lectures, 23:9 (A.D. 350). 
