I thank thee, Father, Lord of heaven and earth, that thou hast hidden these things from the wise and understanding and revealed them to babes; yea, Father, for such was thy gracious will

That is a very reasonable question, considering that all Christendom has honored the apostolic epistles and Paul for almost two thousand years.  How dare me!

There is so much to say -- so many reasons for my stance -- that I cannot give a proper or adequate response in a few words.  Have you read Paul: the Stranger? That would begin to answer your question, but I will not leave you with that.  I will attempt to explain as briefly as possible.

The short answer is: Because Jesus says so!

This is not something I just sat down and dreamed up one day.  I have been a disciple of Jesus for 57 years and am now near age 78.  The early years were spent in the Southern Baptist ministry in growing stress as I sought to make the gospel that I was preaching correspond to Jesus as I was coming to know him through my study of the gospels.  This contributed to a major breakdown (breakthrough)  in 1967, during which I received a wonderful word of affirmation directly from the Lord.  I began to heal and since then I have not looked back.  Yet it was not for another twenty years that Jesus began to come into focus, so thoroughly indoctrinated had I been with Christian doctrine.

The key is to understand that Jesus spoke the Words of God as no other, and those Words are the only authority we have or need.  When we listen to them carefully, without having their meaning screened by Christian doctrine, they present a clear and concise message that gives meaning to the world in which we live, with all its hatred and violence.  They make sense of the world and of my life and experience in the world and give me purpose.  That is the bottom line.  I didn't just decide to believe in Jesus and his Word, but turned to him earnestly seeking enlightenment that would give me understanding of the existence of war, poverty, racial hatred and all kinds of injustice such as we face in this life.  In 1947 I was a youth in the US Navy ready for combat, and had grown up in a community where racial segregation spawned injustices of all kinds.  Jesus seemed to be speaking to these things in the Sermon on the Mount, with which I was then familiar.  I was drawn to him and so I believed.  He has made me free, and so I continue to follow him in his Narrow Way.  Thanks to him, I now understand the world, its purpose and my purpose within it, and have the hope of glorious eternal life beyond it.

Your question suggests that there is added value in believing the NT scriptures outside the gospels -- something to learn that is lacking in Jesus.  I have not found this to be the case, for his Word is perfect and complete, lacking nothing but sincere believers.  To keep this short, I will here quote him to illustrate what I mean:

John.8 [31] Jesus then said to the Jews who had believed in him, If you continue in my word, you are truly my disciples,
[32] and you will know the truth, and the truth will make you free.

For knowing the truth, his word is sufficient.  We need no other.

For freedom, his word is sufficient.  We need no other.

Mark.13
[22] False Christs and false prophets will arise and show signs and wonders, to lead astray, if possible, the elect.
[23] But take heed; I have told you all things beforehand.

He omitted nothing, but many seek to lead us astray. It might be useful to go to the following link and see every instance in the gospels where Jesus uttered the phrase, "my word."

Jesus, in speaking of "my word" was not speaking of the Bible, but only of the words he uttered, which he heard from the Father.  Here is where he makes a careful distinction:

John.5
[39] You search the scriptures, because you think that in them you have eternal life; and it is they that bear witness to me;
[40] yet you refuse to come to me that you may have life.

I do not belittle his true apostles; it is due to them that we have his words preserved in the gospels -- and they would be the first to tell us that they have nothing to add to what he taught them.  But the simple fact is that there are contradictions between the clear word of Jesus and most of the NT Epistles.  Almost all, except Jude and James, falsely interpret the crucifixion as a sacrifice of atonement with Jesus as the substitute lamb of God bearing the sins of humans. This is the doctrine of Christianity; it is not the doctrine of Jesus.  By listening to him, one gets a completely different gospel and interpretation of the crucifixion, such that only by listening to him can one attain to salvation and eternal life.  Once one comes to understand this, then one sees Paul, whose epistles were written first, as the founder of Christianity, which he based on utterly false doctrines that entrap souls in death rather than bringing them life and freedom.  We should all know better, because Jesus pointedly said,

John.5
[24] Truly, truly, I say to you, he who hears my word and believes him who sent me, has eternal life; he does not come into judgment, but has passed from death to life.

Our eternal destiny hangs in the balance.  Will we listen to Jesus -- take him seriously -- or will we follow the Christians in listening to Paul and others -- the NT epistles, Revelations, Luther, anybody other than Jesus? 

John.10
27] My sheep hear my voice, and I know them, and they follow me;
[28] and I give them eternal life, and they shall never perish, and no one shall snatch them out of my hand.

Jesus only! It's all there -- perfect, complete, consistent, beautiful.  And, we have had these words before us all our lives in this world, so we have no excuse. 

His true apostles are the first ones to tell us that we need no one to teach us other than Jesus.  Here we have this stated clearly, understanding the anointing is that of the Word of Jesus and the Spirit that bonds with the Word and teaches us:

1John.2
[26] I write this to you about those who would deceive you;
[27] but the anointing which you received from him abides in you, and you have no need that any one should teach you; as his anointing teaches you about everything, and is true, and is no lie, just as it has taught you, abide in him.