Because they did not believe this fairly represented the voters. For example one state would have one representative for 50,000 people, but another state would have one representative for 300,000 people. They believed the larger state should of had 6 representatives so that every 50,000 people have equal representation.
On the other hand the smaller states believed they would have had less power in a federal government if they only had one representative compared to a larger state with six. So that is why the Senate was created so that every state would have equal representation regardless of population.
