This war was fought because of differences over what England's political system should look like. At issue was the relative amount of power that was to be held by the monarchy and by the Parliament. On the one side of the war were the "Roundheads" who believed that the Parliament should have more power than the crown. On the other side were the "Cavaliers" who believed that the crown should remain dominant. The English Civil War was fought between these two sides with the supporters of Parliament, led by Oliver Cromwell, eventually coming out victorious.
The people couldn't agree on whether or not parliament or monarchy should have more power, and because the people disagreed the civil war took place.