They did it because they feared that without a specifically enumerated bill of rights that the government could become tyrannical in the same manner in which the British were tyrannical. They believed that rights of people and states should be written and stated officially instead of just being implied since implied things are easy to exploit and maneuver according to your needs.
Founders who supported natural rights also supported adding the Bill of Rights to the Constitution because they were fearful that the federal government would become too powerful and therefore tyrannical.
