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Patricians and Plebeian joined together in order to overthrow the king, and after that new laws were created. Unfortunately, these laws was good only for Patricians, so most power fell in the hands of patricians.
Also there were economical reasons. During war Patricians could hide behinde the walls, so they were very well protected. On the other hand plebians often had to leave their farmers in order to take a part in the war. At the and of war