The gospel of Barnabas, however, has absolutely no apostolic support and was written 1400 years after the time of Barnabas. This is evidenced by the fact that it was never quoted by any church father or church historian before the 16th century! Even had the gospel of Barnabas been written during the time of the Apostles, it still would have never attained canonical status due to the historical and doctrinal errors it contains. For example, the gospel of Barnabas purports that Jesus did not claim to be the Messiah (see Matthew 26:63-64). The gospel of Barnabas also says that Jesus was born when Pilate was governor (but history records Pilate becoming governor in A.D. 26 or 27). Furthermore, the gospel of Barnabas keeps some strange company, for it's a favorite among Muslims as it teaches a Jesus consistent with the Koran. The gospel of Barnabas claims that Jesus did not die on the cross, as does the Koran in Sura 4:157. Historians are unanimous that the gospel of Barnabas was written in the 15th-16th century A.D., most likely by Muslims seeking to discredit the Biblical message regarding Jesus.http://www.gotquestions.org/gospel-of-Barnabas.html
The reason why most Christians do not follow the book of Barnabas is because it was recorded later than the other gospels. Some Christians are also of the idea that it contains some Islamic doctrine. Some Christians also question the inspiration and motive that the author of this book had.
The book of barnabas is one of the appocryphal books considered by most christians not to be a legitimate book of God tho it may be an excellent history book. The real Bible is considered to be the true inspired breathed words of God himself written by men He chose to write the books. This was determined by an international, inter-denominational comittee of scholars who did extensive research and decided on which books had enough cross refence proof to be of God and which didnt. The many appocryphal books including the Book of Barnabas didnt make the cut.
It is not part of the Holy Bible. It is written: 2 Peter 1:20-21 But know this first of all, that no prophecy of scripture is a matter of one's own interpretation, for no prophecy was ever made by an act of human will but men mover by the Holy Spirit spoke from God. 2 Timothy 2:7 Consider what I say, for the Lord will give you understanding in every thing.
For the simple and obvious reason that "it doesn't suit their agenda". It will be remembered that there were a great number of Books, of Gospels, Epistles and Acts, and that from these the "inspired" ones, all the four gospels that exist today, were ELECTED BY VOTE by "uninspired" men. The truth is that the Protestants didn't agree as to what books are "inspired" until 1647, by the Assembly of Westminster.