Short answer, it’s because we learn more about the languages that compose the Bible, as well as find new manuscripts, that allow for a better translation.
Long answer: One of the best books on this subject is Bruce Metzger’s The Text of the New Testament. It does get dense though. One of Metzger’s students, Bart Ehrman, has written more popular works that deal with the subject a bit, such as Misquoting Jesus. Ehrman also has a more in depth work titled Studies in the Textual Criticism of the New Testament, which is a collection of articles and lectures he’s made on the subject.
But let’s bring it all together. So the definite text for the Hebrews is the Masoretic text. This text is also used by Christians and Bible scholars. The King James Version relied on the Masoretic text for their Old Testament, and the Masoretic text continues to be one of the primary sources that Bible translators rely on.
The Masoretic text has changed through history though. We don’t have just one example of the Masoretic text, but a variety of manuscripts that contain this text. Some are better than others, and there are differences between the manuscripts. This has led to different editions of the Tanakh, or the Christian Old Testament. One of the early important versions was the Mikraot Gedolot, which is often called the Rabbinic Bible. This was published in about 1516, and the King James Version of the Bible would use it for the Old Testament. But it had thousands of technical errors. They didn’t really change anything, but they also weren’t correct.
The version I use, which is the Jewish Publication Society’s edition, is probably the most widely read today. But in the Jewish Study Bible, as part of their introduction, they mention how this edition, or translation of the Masorectic texts has changed. The JPS began in 1955, and was completed in 1982. It was then revised in 1985, and then another revision and corrected edition (the second edition or the NJPS) was released in 1999.
As the Preface to the JPS explains, the reason for this new translation was deemed necessary, largely because of the advances in scholarship. In order to produce the JPS, they looked not only at the Masoretic text, but also early Rabbinic and medieval Jewish commentators, grammarians, and philologians. Part of it too was just to update the language to a modern English that didn’t use obsolete words.
Now, if you know Hebrew, you’ll just go to the Masoretic text and the translations aren’t needed. But as I mentioned, even the text of the Masoretic text has changed in small ways. It’s a part of textual criticism. As new manuscripts are found, they are compared to other manuscripts, dated, and sometimes used in order to help get to a more accurate portrayal of the original work.
But the nice thing about the Hebrew text is that there is one. It changes as new information is found, but the changes are minor and usually don’t amount to anything serious. It’s something textual critics find interesting, but outside of that, not really. The Christian Bible does also benefit from the Masoretic text, to a point.
Between 1947 and 1956 a massive discovery was made; the Dead Sea Scrolls. Geza Vermes has done excellent work on this subject, including his book, the Complete Dead Sea Scrolls in English. They gave us the earliest copies of various books within the Tanakh. And there are some differences between them and the Masoretic text. When it comes to new translations of the Christian Bible, most will not only rely on the Masoretic text, but they also utilize the Dead Sea Scrolls. The discovery actually pushed for a number of new and revised translations, including two of my favorite, the New Revised Standard Version and the New Jerusalem Bible.
There have also been a number of discoveries when it comes to New Testament texts. Metzger does a terrific job of going through this. We can thank Johann Jakob Griesbach (who lived from 1745-1812) for kind of revolutionizing the manner in which the Greek text of the New Testament was worked on. When thinking of the historical transmission of the New Testament, we aren’t just talking about Greek. The works were also translated to Gothic, Coptic, Armenian and Syriac, as well as other languages. And up until Griesbach, those manuscripts weren’t studied as much. But Griesbach brought them into the fold as he studied the manner in which the transmission of text occurred. He really helped found the scientific method of textual criticism.
Through all of this, we learned how to determine which texts were better, which ones were more accurate. This in turn effected how the New Testament was translated as there was a push for more accurate translations.
Most translations are based on the best manuscripts we have at that time, but we continually find new manuscripts, or portions of manuscripts, that are then compared. For instance, the King James Version of the Bible utilized the Textus Receptus. That was the standard at that time. But the Codex Vatinicus and Codex Siniatucs have been found to be older and better (part of the problem with the Textus Receptus is that it was based on Latin, that was then translated to Greek, that was then translated to English).
After finding those manuscripts, scholars do have to address possible corruption, and if possible, try to restore them. Which takes time and can be quite hotly debated. But it also produces better base manuscripts to translate from.
It’s not just the matter of finding better manuscripts though. Koine Greek, which the original New Testament texts are written in, is a dead language. So understanding the language can be difficult. And sometimes our understanding is forced to change. Around 1898, a group of largely Greek manuscripts were found in a rubbish dump near Oxyrhynchus in Egypt. There are around half a million papyri that were found, and about 5,000 have been pieced together and transcribed.
In 1952, E.G. Turner wrote an article titled Oxyrhynchus and Its Papyri, where in just about 5 decades, those papyri had already had a massive impact on classical studies in general, and the understanding of Greek. That’s almost 70 years ago, and they continue to make a big impact. That also forces scholars to rethink the Greek language at times. Interestingly enough, this also means that if you are taking ancient Greek, you may have professors who even pronounce Greek words differently because we have a better understanding today of how Greek was pronounced.
So coupled with finding new manuscripts and then reworking the Greek language, new translations come out to keep up to date with these findings, and to give a more accurate reflection of the original text.
There is a bit more as well though. Metzger in The Bible in Translation points out that some of the translation are theologically motivated. So with the Revised Standard Version, which was meant to update the King James Version, some believed it was too liberal, and thus we got more evangelical based translations. One of those would be the New International Version.
Some of the translations also just attempt to read easier, so they are much more of a paraphrase of what the text says. Other times you have various denominations seeking to have their own translations, and sometimes that just means having the deuterocanonical books added in the right order.
So the reason we have so many translations is two fold. First its to keep up with modern scholarship as we find better manuscripts, learn more about Greek, or simply because the language is outdated and needs to be updated. Other times its because there is a theological motivation for a new translation, whether that be to add additional books in the right order, to make sure the texts fit their theological views, or just to make the text simpler to read so it can be spread easier.
Which one is the accepted version? It depends. Evangelicals tend to go more with the New International Version. Its a decent translation, but at times it chooses a translation of certain passages more because of the theological view of the translators. The New Revised Standard Version is one that is often seen within mainstream scholarship. It was produced by a diverse committee of scholars, and it is often agreed that it’s based on the best scholarship we have right now. The New Jerusalem Bible was once a major Catholic Bible, especially with French Catholics. That just has to do with who it was produced for.
So there is no officially accepted one. Different groups prefer didn’t versions though.