The Declaration of Independence does not directly address the issue of slavery mainly because the delegates chose to avoid a divisive fight over slavery in order to keep all thirteen colonies united in the struggle for independence from Britain. Many delegates, especially from Southern colonies, were slaveholders or economically tied to slavery, so a direct condemnation of slavery risked splitting the Congress and derailing the bid for independence at a critical moment. In other words, even though Thomas Jefferson’s original draft included a strong passage criticizing the slave trade and blaming King George III for supporting it, that section was removed after intense debate. Delegates from slaveholding colonies (and some from Northern colonies involved in trade connected to slavery) pushed to strike it out so that the final document would be acceptable to all the colonies and preserve political unity against Britain. So the “best” explanation you’re usually expected to give in a class or test setting is:

Because the authors feared that directly challenging slavery would divide the colonies and weaken support for independence, they removed the passage and left the issue for later.