Why do people think they need anything other than the KJV Bible?

The answer to this question is in the wording of the question itself - it should have worded something like:

I canst not apprehend the thoughts of man beseeching why needest thou more translation of holy writ. What thinkest thou?

That is, even the questioner does not use Tudor English with its obsolete word declensions and verb forms. Many of the words now mean something different from what they were originally intended as well.

The other major reason concerns the change of text which the KJV used - it contains numerous additions not in the Greek that were added from the Clementine Vulgate.

Finally, the KJV of 1611 that some so prize is NOT the original KJV but its 5th revision from 1769. We should continue to revise as the need presents itself.

Being the "Authorised" bible it can lend itself to being thought of as The Bible. However it was only Authorised for the Anglican Church by the King of England: James I So from a Catholic (and Orthodox) perspective, it hasn't been approved for us or by us.

Again from a Catholic perspective, it's not a complete bible; Catholic bibles are traditionally based on the Septuagint rather than the Masoretic (although we do use it as a textual source).

The Septuagint contains more books that are considered to be part of canon, called Deuterocanon or Apocrypha by other Christians so our OT would have 46 books and the KJV would 39 if my maths is correct.

The KJV that we have today is not the original KJV Bible as it would be hard to understand., example John Ch 17:-

Viewing the original 1611 KJV with archaic English spelling. Go to https://www.kingjamesbibleonline.org/1611_John-Chapter-17/

John Ch. 17:1 These words spake Iesus, and lift vp his eyes to heauen, and said, Father, the houre is come, glorifie thy Sonne, that thy Sonne also may glorifie thee.
2 As thou hast giuen him power ouer all flesh, that he should giue eternall life to as many as thou hast giuen him.
3 And this is life eternall, that they might know thee the onely true God, and Iesus Christ whom thou hast sent.
4 I haue glorified thee on the earth: I haue finished the worke which thou gauest me to doe.
5 And now O Father, glorifie thou me, with thine owne selfe, with the glory which I had with thee before the world was.
6 I haue manifested thy Name vnto the men which thou gauest me out of the world: thine they were; and thou gauest them me; and they haue kept thy word.

So as language changes so does The Bible text so people can read it.

The KJV is only a translation and not as good as newer ones because our understanding of Hebrew, Aramaic, and Greek has improved over 400 year plus when the KJv was produced, thus most (not all) reflect a better understood text which is also more accurate then the KJV was produced in 1611!

Many people use the KJV in order to read a translation of the Textus Receptus, the Received Text, rather than the Westcott & Hort/Nestle-Aland Greek Text which gives great preponderance to the Codex Sinaiticus and the Codex Vaticanus.

Most, today, will be using the 1769 translation of the KJV rather than the 1611, whose English is very archaic.

Good as the KJV is there are some translation issues which need to be addressed, such as the use of the word 'atonement' in Romans 5:11 for καταλλαγην , katallasen, which ought to be rendered as 'reconciliation' or 'restoration'.

Similarly in Romans 5:18 δι ενος δικαιωματος has been translated 'by the righteousness of one' which is a false translation and it should read 'by one righteousness'.

Other necessary corrections could be listed, particularly the habit of the KJV translators adding the word 'man' when there is no anthropos or aner in the text.

There is a 2016 update but I cannot comment as I do not use it. Personally, I await the publication of what is currently being called a 'More Accurate Translation'.

But if one wants absolute accuracy, one has to go to the original language. It is the Greek that is inspired, not the English.

Textus Receptus Bibles

There are many reasons, but here are two:

Most Christians don't speak English. This is obvious, but there are millions of Christians around the world who can't understand even basic English. Asking them to use the King James Version is folly, which is why there are Bible translations in every major language and why Bible translators work tirelessly to translate the Bible into minority languages.

Most English-speaking Christians don't understand KJV English. It is well documented that very few English speakers correctly interpret the KJV, even those who think they know what they are doing. This goes far beyond "thou" and "thee" – there are numerous words whose meaning has changed significantly, but which look familiar to us.

Mark Ward, author of Authorized: The Use and Misuse of the King James Bible, gives dozens of examples of these "false friends" in his book. One example is 1 Thessalonians 5:22:

Abstain from all appearance of evil. (KJV)

We think we know what "appearance" means, but actually, to the KJV translators, the primary meaning of the word was "instance," not "outward aspect." This is why modern translations generally translate the verse along the lines of "Abstain from every form of evil" (ESV).

There's a lot more that could be said, but this gets to the heart of the issue. As William Tyndale said, one of the purposes of Bible translation is so that even the most uneducated – even the "boy who drives the plow" – can understand what he reads.

All languages change over time, all aspects of it are subject to change from basic orthography to fundamental meanings of words.

It is highly unlikely that even you are actually using the KJV as published in 1611. Given the spelling and choice of words used in your question, you don't speak King James era English, you speak something quite a bit more modern. In all likelihood you are using a 19th century update known as the Cambridge Edition that revised much of the spelling and punctuation.

However that edition is still fundamentally based on many of the translations choices of the 16th century. While many of the words used were good choices based on the usage of their era, the English language has changed and properly representing it to speakers of modern English requires a translation in their language. Many/most of the other translations in common usage were made to address this.

There are of course other reasons that come in to play. Some translations are made because the ones people would otherwise use are under a copyright that restricts their use. Others are made with differing theological or cultural agendas that they think would better represent the Bible.

At the end of the day the answer is usually the same for any language and translation. Why use the KJV, why not use Geneva Bible? With the Geneva Bible and the Tyndale NT around why did anybody even feel the need to start the KJV project in the first place? For that matter why not use the Vulgate? Why not just teach everybody Hebrew and Greek and read it in the original?

The reason we have translations at all is to make the Bible understandable.