That's like asking, "why are there so many different English versions of 'The Iliad' if there's only one English language?"
But, if you need a more complete answer, read on:
1) Language - modern language keeps changing. Some words still in use today have very different meanings than they did in the time of the King James Version (KJV), for example. Witness how, in the KJV Old Testament (OT), the word "meat" is used to refer to bread and the word "bread" is used to refer to meat! This is just one of the most obvious of a multitude of such changes in meaning. Modern translations ensure comprehension by modern readers.
2) Source texts - archaeologists are continually making discoveries of more ancient, and more authentic, source texts. There is *still* no version that takes full advantage of the texts among the Dead Sea Scrolls, for example, and they were first discovered over 50 years ago. Modern, scholarly translations take advantage of the most recent and most authoritative source texts available.
3) Scholarship - the knowledge of translating the ancient source manuscripts increases with time. Thus, a modern, scholarly translation is bound to be more consistently accurate than a scholarly translation of 400 years ago, or even 100 years ago.
4) Translation methodology is important. Style ranges from a word-for-word literal translation (which tends to inaccuracies but is useful in study), to a thought-for-thought translation (which tends to be accurate but often displays over-interpretation as well as translation). There is a world of "gray area" between these two extremes.
5) Content - though some versions strive for an "inclusive" canon, i.e. a bible that contains all books used by all major Christian sects, most display a very strong religious bias by including only a more restrictive canon. This can be seen by comparing the contents of the New American Standard Bible (restrictive canon) with the Oxford Annotated New Revised Standard Version (inclusive canon).
6) $ - this is the most significant contributor to the many versions available today. Any publisher that produces a modern, scholarly translation can expect to earn a profit from publishing that work. Thus, publishers are continually producing such works in an effort to fill a niche in one of the above areas or simply in offering an improved (modernized) bible to replace a currently-filled niche. If producing a new version were a money-losing proposition, only a very few well-funded religious or university organizations would be making the financial effort to produce such works. We would likely have no more than 4 new translations every century under such cost-prohibitive conditions.