Because Joseph Smith is a proven liar. I don't mean to be disrespectful toward other religions in the Religion forum, so my apologies to any Mormons reading this. But nothing gives Joseph Smith divine authority, and he has been shown to engage in fraud.
He claimed to have gotten the book of Mormon on Golden Tablets from an Angel named Moroni - who took the Golden Tablets back.
Well... ok.
Even if I were to accept that, there's more. After starting his religion, he got a hold of some Egyptian papyri which he "translated" and said were the lost books of Abraham and Joseph. These papyri held such interesting tid-bits such as the statement from God that says that black people were supposed to be servants and slaves to whites and Asians.
With the discovery of the Rosetta stone, it was determined that, instead of being the Books of Abraham and Joseph, the documents were actually copies of The Egyptian Book of the Dead and The Breathing Permit of Horus.
So I put little credence into Mormonism.
I'm fairly certain the bible was, in fact, completely written and assembled in 1829.
Anyway, just because someone says they're inspired by God doesn't mean they ARE.
I've already spelled out the reasons I believe in the Bible, and I see no reason to hash that all out again - but since I DO believe in the Bible, I believe it is inspired by God - and I believe that it's the ONLY word of God.