Because they're the two biggest news sources?
CNN is chronically last. FoxNews is invariably first.
On TV (amongst the big three news channels), Fox is first, then NBC, then CNN. https://www.yahoo.com/entertainment/124-cable-channels-ranke... https://en.wikipedia.org/wiki/Big_Three_(American_television...
Furthermore, opt-in is not a conservative trait hence its inherent bias in favor of those who participate actively to the YouGov data farm.