Why did the Lamanites become a dark, filthy, and loathsome people?
This shall be a law unto you, that ye receive not the teachings of any that shall come before you [besides Joseph Smith]
as revelations or commandments. Doctrine & Covenants 43:5
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