The framers used compromise in order to increase the chances of the constitution being ratified. Each of the political factions developing during this time, the federalists and anti-federalists, had different things they wanted in terms of structuring American society.
The federalists wanted a new constitution and to get rid of the Articles of Confederation. The anti-federalists, wanted to simply amend the Articles of Confederation. However, the anti-federalists were open to a new constitution as long as it had a list of individual rights that the government could not take away.
The framers compromised and gave each side something they desired. The federalists would get a new constitution and the anti-federalists would get a list of rights they were guaranteed to have. This became known as the Bill of Rights.