A major reason that Joseph Smith's polygamy is not a part of standard church manuals is because the fact was in doubt historically for many years because Emma insisted until her death that no polygamy had occurred. Because of this open question, the church refrained from taking a position. The historical record has now made it quite clear that Joseph Smith had many plural wives, but the lack of discussion in lesson manuals has much more to do with the church waiting to ensure they have an accurate history, than trying to hide elements of its history.
