AFAIK the the writers of the NT are only Matthew, Mark, Luke, John, Paul, James, Peter and Jude.
Why only ? Perhaps things might make more sense if, instead of focusing on the writers, one would avert their attention towards their respective audiences.
There were two main social, political, and religious groups around the time of Christ: Jews and Gentiles; Matthew writes to the former, while Luke addresses the latter.
Hellenistic Judaism acted as a bridge between the two, and it is these that John tries to co-opt. His gospel is also highly philosophical, and thus relevant for the intellectual elites of its time; which is where Mark comes into play, since his work is primarily aimed at reaching the common people. (A certain British song comes here to mind...)
These Gentiles and Hellenized Jews then needed to know, on a very practical level, what the Christian life would entail for them, which is where Paul's letters prove particularly helpful. But, if Paul speaks for the Seventy, who then speaks for the Twelve ? Well, none other than the three pillars of the nascent Christian community, whom he mentions in Galatians 2:9, namely James, Peter, and John, whose epistles follow immediately after his.
The writings of the other apostles, Andrew, Philip, Bartholomew, Thomas, Matthias, Simon and James, why are they not included as a gospel of the Bible?
Ultimately for the same reason the works of Shakespeare are not included in Romanian literature...
The Apostles were living, breathing human beings, who've founded the earliest Christian communities, and it is to these flesh-and-blood communities that they also left their authentic writings.1 Though pious individuals (either Christians or members of syncretistic faiths, such as Gnosticism) composed works attributed to various venerable figures, there were no real-life communities, founded by the actual apostles, to have these writings in their possession, be able to vouch for their veracity, and consequently pass them on, through their descendants, to successive generations; which is also why, until this very day, they are still not found in our possession.2 Though technically (and rather ostentatiously) bearing the names of illustrious Christians, their status is, in practice, no different than, say, that of the Qur'an, for example.
1 This is especially transparent, for instance, in the last verses of the fourth Gospel (John 21:24), where the voice of the anonymous narrator melts together with that of the community he shepherded. (See also 3:11-12, and notice the usage of the plural pronouns we and you).
2 I am not saying that we are unaware of their existence, or unable to either buy or consult them, but they are not inherited by us from our predecessors in the faith, by way of an unbroken and well-attested historical transmission, in the same way in which the other biblical and patristic books are.
And who is the author of Hebrews in the NT?
On one hand, there are people who would want us to expand the canons of either Testament to include a plethora of pseudepigrapha, deeming our traditional list way too small, while, on the other hand, there are biblical scholars who would like us to exclude from the Bible books whose authorship has never been seriously questioned, historically speaking, by the earliest Christian witnesses.3
Not that the Letter to the Hebrews is among the latter, but I thought it best to point out this discrepancy; indeed, its authorship has been brought into question since antiquity, as mentioned by Eusebius of Caesarea in the third and twenty-fifth chapters of his third book on Church History. As can be glimpsed from the aforementioned passages, there seems to have been, since ancient times, a solid core of about twenty undisputed books to the New Testament, followed by a certain gray area of otherwise sound and respectable books of somewhat doubtful or uncertain authorship, providing some leeway to the possible extents of the Christian canon.
3 When attacked from two opposite sides, it is best to just stay quiet, and let the two sets of (ideological) adversaries obliterate each other's arguments, as Saint Paul did in the Book of Acts (23:6-10).