Because lds put their “book” above the Bible says all you need to know.
Too bad they don't actually FOLLOW it!
"Now if any of you will deny the plurality of wives, and continue to do so, I promise that you will be damned;
and I will go still further and say, take this revelation, or any other revelation that the Lord has given,
and deny it in your feelings, and I promise that you will be damned.
Brigham Young - JoD 3:266 (July 14, 1855)