The Mormon escape clause on believing "the Bible to be the word of God" states that the Bible should only be trusted "as far as it is translated correctly" (Article of Faith 1:8), so one might assume that Mormons have undertaken a very rigorous translation effort and that the Mormon version of the Bible is one of the best around.
The reality is that the LDS Church has stuck to using the King James translation, and the King James translation of the Bible actually kind of sucks. It's archaic and difficult to understand, it's full of mistranslations, and the translation was extremely limited to the texts available in England at the turn of the 17th century.
So why the King James Bible? Maybe because that's the version the Book of Mormon prophets preferred.