By placing the translation condition on the bible and NOT on the book of Mormon, at least two things should be evident:
1. That the book of Mormon is held in higher authority against the Bible
2. That they are aware of the contradictory nature of the Bible vs the BOM.
And I really don't know WHY.
NONE of their SACRED rituals so proudly and purposefully done in the Temple are even FOUND in the book!
They could get rid of it entirely and NOTHING would change in their practice of their religion.