Why did the Lamanites become a dark, filthy, and loathsome people?
We do not believe that human law has a right to interfere in prescribing rules of worship to bind the consciences of men, nor dictate forms for public or private devotion Doctrine & Covenants 134:4
Why did the Lamanites become a dark, filthy, and loathsome people?
Because of their unbelief and idolatry
This people [the Lamanites] shall be scattered, and shall become a dark, a filthy, and a loathsome people ...
because of their unbelief and idolatry. Mormon 5:15