I conclude this provision was included to weaken the national government. Such a super-majority as 9 of 13 states needed to make laws would seriously slow the political ability of the central government, making it very weak.
The Articles of Confederation included this provision in order to add restrictions to the state's freedom without entirely taking away their rights. This would also prevent single states attempting to affect/change the country, as they needed all the states to agree. This would have handicapped the central government by adding even more reasons for states to conflict with each other.
