Because at one point Britney, success and performance wise was everything Beyonce wanted to be. They were peers and led the pack. Then post 2003 Beyonce, a black woman, represents everything Britney used to be. However Britney's sales still trump Bey's so until she eclipses her overall the war will never end. Hence why many army members hide behind Rihanna and use her as a weapon to drag Beyonce since Britney can't be used anymore.