Why did the Lamanites become a dark, filthy, and loathsome people?
Ye unbelieving, turn ye unto the Lord ... that perhaps ye may be found spotless, pure, fair, and white, having been cleansed by the blood of the Lamb. Mormon 9:6 Mormon 9:6
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