Why didn't some people want to ratify the constitution?

There were many concerns over including a document that would protect the rights of the people (The Bill of Rights), in order to prevent another authoritarian government. There were also many issues that several states needed to flesh out in order to come to an agreement. Many states feared that the federal government would be too powerful. There were two groups that debated over these issues: The Anti-Federalists and the Federalists. The issues were fixed by coming to an agreement and including the Bill of Rights. Madison was one of the federalists who persuaded a number of people how the federal government would be effective. -- Keep in mind that these problems were in the 18th century, before many advanced communications and some states were not even included in many discussions which took place in Philadelphia because they were not able to be contacted.