The main issue several states had with the Constitution was that there was no Bill of Rights included to protect the freedoms of the individual. We had just fought a war against Great Britain. One of the main causes of the war was the colonists believed the British government and the King had too much power. When the Articles of Confederation was created, the federal government was deliberately given little power and was a weak form of government.
When the Constitution was written, the federal government was much more powerful. The writers realized it was a mistake to have a weak federal government like the one created by the Articles of Confederation. This increase in power alarmed many of the states as well as the citizens of these states. They were concerned that individual freedoms would be lost. Since this was one of the reasons why we fought the Revolutionary War, there was a push to add a Bill of Rights to protect these freedoms. Several states said that without a Bill of Rights, they wouldn't vote to ratify the Constitution. Thus, a promise was made to add a Bill of Rights later.