Why did the Lamanites become a dark, filthy, and loathsome people?
The first flesh upon the earth, the first man also; nevertheless, all things were before created; but spiritually were they created and made according to my word. The Book of Moses 3:7
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