User blog comment:TheTobarMethod/League System Thoughts/@comment-3541335-20130201150909

This system is more detailed, but the major flaw in it is that it's far from your "true" display of skill. It doesn't show whether I stole the enemy's Baron, my backdooring was the key to victory, or if I distracted the enemy long enough so that my teammates could clean up.

Basically, there's too many different possible circumstances that can't be shown by sheer numbers. Only counting win/loss makes it more likely that you won't wrongly drop down.