Why do you think the founders did not include the declaration of independence within the constitutional plans for the new federal government

The Declaration of Independence was not included into the constitutional plans for the new federal government because is was not an operational document or law. It contains nothing relevant to powers and responsibilities, procedures, or anything else that's needed in order to set up a government; it simply was a letter from the Continental Congress to King George III notifying him that they were about to commit treason by leaving the British empire and creating their own country, and listing their reasons for doing so.