If a group is left-leaning, they'll report on everything good about the left and everything bad about the right, If a group is right-leaning, they'll report on everything good about the right and everything bad about the left. For example, you'll almost never see CNN write an article critical about Democrats, and you'll almost never see Fox News write an article critical about Republicans. So you end up without much overlap between the two.
They'll only report on the same things when those common things are important enough / loud enough to where they can't ignore it, or when they're able to put their own political interpretation on it when telling the viewer what to think.