Why are the Gnostic Gospels not a part of the Bible?

Most of the issues with including certain books into the Bible were questions of authorship. If a book, no matter what it said, was thought to be of dubious origin (by most people), then it was not included.

Several books in the Bible were close to being chopped out. These include Hebrews and Revelation, which eventually did make it into the canon.

The Gnostic gospels were rejected primarily because of these authorship issues. Most of the gospels that you cite (Thomas being the exception) were penned well into the 2nd and 3rd centuries, and so could not have been written by the proposed authors.

Thomas is a unique exception. At least part of it seems to be genuinely written in the 1st century. The Jesus presented in Thomas stands in stark contrast to the Jesus presented in orthodox gospels like John. In the case of Thomas, it was excluded because of some doubts of its authorship, but probably more likely that it was of very limited use in the ancient world (primarily in Coptic speaking areas) as well as its generally lack of harmony with the other canonical gospels, primarily due to its treatment of the resurrection.

Constantine convened the bishops at Nicene to construct the Canon, or book of authorized works to be included in the new testament. these texts were left out as they did not support the dominant thesis that the priesthood is pre-eminent, and the other Christian beliefs are subordinate to bishopric authority on matters of faith and morals. I mean, a gospel authored by a woman!? That would be a no go even if it came up for consideration today! Lastly, the gnostics it was feared were closet resurrection deniers, Yes, I said it, resurrection deniers. Don't want any of that among our holy books!

Written too late
Not written with authority
Never quoted by apostles
Never accepted by 1st Century Christians

from my research, they are mainly written during later times (2nd-4th AD as opposed to closer to when Jesus was alive) and may not be as valid. there are also some concerns regarding their validity and consistency according to the canonization of the Bible. you may be interested in this article: http://www.rbc.org/questionsDetail.aspx?id=46126

They were deemed non-canonical by many wise and capable men who were led by the Holy Spirit in their efforts.

They were not written through yielded human instruments led by God the Holy Spirit, that's why. All of scripture is the inspired word of God, and these are not part of it.

Various reasons. The canon was restricted to books written right away. Books obviously written in the second century, even if widely approved were excluded.

Scripture was not put together to be an unbiased survey of what everyone in the world thought about Jesus. Because the Apostles had died and almost all eye-witnesses were dead, it was thought important to have a written record of their recollections.

The first scriptures proposed by Marcion and he was too radical for the rest of the Church. He rejected the Jewish scriptures that were used for prayer and worship in almost all Christian communities. The canonical scripture collection was a statement against what he and others, including the gnostics taught about Jesus.

Just as today we have new age Christianity that combines Jesus with other gods and Chrisitian practices with magic and new beliefs, so too did the Greek mystery religions and pagans add Jesus and his God to their pantheon. While the Greek mystery religions provided a framework for communicating the teachings of Jesus, there were elements that most Christians agreed should be excluded from Christianity.

Scripture defined Christianity to be what it was believed by most of the people and their leaders to be. Just as today, many issues were not definitively settled and a wide variation of belief was tolerated. Just as today, teaching that cross the line were condemned, and creeds and oaths written to publically teach what was and was not acceptable to Christianity.

Scholars find that many of the saying of Jesus in the Gospel of Thomas are athentic. It did teach things that were accepted to culture of the time that was unacceptable to Christianity. It didn't take much to exclude something. A hint that Jesus was not divine. A turn of phrase that could be take that Jesus was not a real human, but more an avatar.

It is amusing that the Revelation of John was excluded from the proposed canon by more than a few of the Church Fathers. It certainly shows a different perspective and is the root of many new strange Christian beliefs.

Fortunately, (for modern practice) the books you list were excluded. Instead of Paul declaring Christianity genderless and classless we would have Thomas saying the women must first become male to be saved.

To weave a conspiracy theory where the true church believed many things and that an establishment brutally crushed for political reasons all other thought is appealing to those who root for the underdog and who are suspicious of the faithfulness of large organizations and their beuracracies, but it does not match the entire picture. That parts are plausible does not mean that that is the way they happened.

You forgot the Gospel according to Peter.
The Gnostic Gospels were not considered authentic enough by Holy Mother Church to be included in the the Holy Bible.

The are not accepted as authentic. Reasons include (depending upon specific the book) supporting ideas contrary to more accepted beliefs, not being as popular, and being far too new to possibly authentic.

The people that collected and decided on the stories for the Bible, didn't feel that they belonged...

And some feel that they were 'of the devil' as much as those chosen were 'of god'... *rolls eyes*