I think it's important to remember why the respective networks were founded.
CNN began as an interesting idea. A channel that just did news 24/7.
Ted Turner looked at MTV - a 24 hour music channel - and said "let's do that, but with news!"
Fox "News" began with a mission of bringing a conservative slant to the news. The entire operation is based on the idea that "the news media" is inherently "liberal," and they were purpose-built as the alleged counter-point. Bias was baked into the cake from day one. The roots of this idea came from the Nixon era...and Nixon was brought down in no small part due to the media exposing his shenanigans. Is it any wonder that the founder of Fox News was an aide to Richard Nixon? Roger Ailes pushed the idea of creating a "pro-administration" TV network back then that would spoon-feed propaganda directly to an easily-manipulated public.
Ted Turner just wanted to jump on the bandwagon of 24/7 cable channels like MTV and ESPN and use news as his vehicle to make money. Ailes finally got the propaganda network he wanted.
The semi-tragic thing is that Fox - by becoming a ratings winner - influenced the direction of CNN and (arguably) led to the creation of MSNBC. They proved that biased news/opinion sells better than straight news. Fox sought to challenge the traditional news, but their model has in many ways supplanted it. Roger Ailes metaphorically killed the likes of Walter Cronkite. Instead of a steady voice relaying the hard news, our prime time is filled with partisans shouting at viewers.