Why are there so many Bible translations? Moreover, why can't we believers in Christ just have one book? I think that Jesus said NOT to do that. I'm pretty sure that this is quoted at the end of the Book of Revelation. Also, why do the scholars or whoever take or omit books, words, and/or passages out of the Bible?
Short answer to your question is...
Why are there so many Bible translations? Because, the original languages of the Bible are very old and no one speaks them anymore. The Old Testament Hebrew and the New Testament Greek are no longer spoken today. Scholars are still trying their best to produce the original meaning but there is always debates and different opinions on how to translate it correctly.
Why can't we believers in Christ just have one book? Protestants have 66 books in the Bible. Roman Catholics have 73. If you compile all these books (66 or 73) into one book and call it The Holy Bible, then of course, we have exactly one book! We have many versions of the Bible because of translation issues.
I think that Jesus said NOT to do that: Only the Book of Revelation mentions that.
Revelation 1:10 (NIV) On the Lord’s Day I was in the Spirit, and I heard behind me a loud voice like a trumpet, which said: “Write on a scroll what you see and send it to the seven churches: to Ephesus, Smyrna, Pergamum, Thyatira, Sardis, Philadelphia and Laodicea.”
Revelation 22:18 (NIV) I warn everyone who hears the words of the prophecy of this scroll: If anyone adds anything to them, God will add to that person the plagues described in this scroll.
God forbids anyone to alter the contents of the Book of Revelation. Here "this scroll" literally means the scroll that John was writing, which we call it the Book of Revelation of Apostle John.
Do we have the original scroll written by John? NO! No original documents of the books of the Bible are available today. Only copies from the original documents are existing today.
Why are there so many Bible translations?
There are multiple reasons for this, there are two separate texts for the New Testament, the one that is used by the King James and New King James has thousands of copies that match or near match, there is another set of texts used by most modern translations like the NIV. The NIV Greek text is dated as older, but is based off only two copies to my knowledge the textus sinaticus and the textus vatinacus, both contain heavy ommissions, the people on the NIV board thought these texts were better because they are dated as older, but an older copy of something does not necessarily make it better. That is why when you read the NIV it is missing some verses.
I actually grew up on the NIV, I sometimes quote scriptures from the NIV, but now I use the NKJV as in my humble opinion it is more literal and the texts for the New Testament are much better. If you get a NKJV Bible that has footnotes you will notice a ton of scripture notes that say something like NU-TEXT omits the word abcd, that shows you the omissions of the relatively small copies used by the modern translations.
Another reason for so many translations is money, everything except the King James Version to my knowledge is copyrighted and if you want to print a version of that Bible you need permission from the original publisher.
Another reason for so many translations is who sits on the translation board. Imagine four different boards, one board is Catholic, another Baptist, another Seventh-day Adventist, another some other denomination, are their interpretations of verses going to be different and affect the way verses are translated? Of course.
One of the major reasons why there are so many translations have to do with textual variations in Greek NT Texts.
Let me take Greek NT manuscripts Codex Vaticanus (B) and Codex Sinaiticus (Aleph) as an example.
According to Greek Scholar Herman C. Hoskier in his book "Codex B and it Allies" (1914) volume 2, there are, without counting errors of iotacism, 3,036 textual variations between Sinaiticus and Vaticanus in the text of the Gospels alone, enumerated as follows: Matthew: 656, Mark: 567, Luke: 791, John: 1022.
Pericope de Adultera (John 7:53-8:11) is not found in earliest Greek NT manuscripts like Papyrus 66, Papyrus 75, Codex Vaticanus and Codex Sinaiticus. It is not in Eastern Aramaic Peshitta NT manuscripts either.
Due to its textual variations and additions in Greek NT, Several English translations were made.
Side Note - Unlike Western Tradition, Several Eastern Tradition (Middle East and South India) use Eastern Aramaic Peshitta as New Testament text. Not Greek NT texts.