Word of Wisdom - Section 89 of the D&C

The Church says that this section 89 of the Doctrine and Covenants is a commandment. My question then is...if it really is a commandment from God why then has their not been an amendment to the D&C? Instead it continues to read that it is was "Given for a principle with a promise" and that it is "To be sent greeting; not by commandment or constraint".