There were several reasons why the NWT was printed:
1) Most of the other translations used were made by those who were influenced by the pagan philosophies and unscriptural traditions that their religious systems had passed down from long ago as well as other influences.
2) Older and more reliable Bible manuscripts were becoming available.
3) As a result of archaeological discoveries, the Greek language of the first century was becoming more clearly understood, and
4) The languages into which translations are made undergo changes over the years. (For example, who today really talks like this?: "And he commanded the foremost, saying, When Esau my brother meeteth thee, and asketh thee, saying, Whose art thou? and whither goest thou? and whose are these before thee?" (Gen. 32:17 KJV)
Jehovah's Witnesses wanted a translation that was of the latest scholarship, one that was without spot by creeds or traditions, a literal translation that faithfully presented what is in the original writings and a translation that would be clear and understandable to modern-day readers.
For more, see
Defending the New World Translation
How does the accuracy of the New World Translation Bible compare to other translations?