Why was slavery omitted from the Declaration of Independence?

In practically relevant terms for American society in the 18th century, the Declaration of Independence ignored the issue of slavery because of the tremendous internal divisions which the Continental Congress suffered from. Many American patriots were dedicated slave-holders, while many were not. The issue was left unresolved until independence could be won. In theoretical terms, however, the Declaration does hold out the truth that all persons in a truly free society are created equal and deserve equality before the law (and in any other significant respect).