Why did states hold constitutional conventions?

They held constitutional conventions because they could never agree on anything for the betterment of the U.S.,  so they had to all have at least 9 votes per states and then get things done that way. otherwise there would be no Bill of Rights.

Explanation:

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presidency should be the title

the virginia plan called for three branches of government and two houses of congress. representation in each house would be determined by population. the new jersey plan called for three branches of government and a single house of congress. each state would have an equal vote.