Because in destiny playing the objective isn't synonymous with being a good team player. How many cap points you have means very little if you can't get any kills. Inversely though kills do matter even if you don't have any cap points. So generalising- a guy with a high k/d and high loss ratio is probably still good but not good enough to offset the rest of his team. It's also why some people can go 6000pts+ on the losing team and still lose because the team can't hold many points.