Breitbart is an editorial/propaganda magazine, not a news source. Why would you go there for anything?
CNN was openly anti-Trump for the entire administration, as in, instructions were literally passed down that reporting should be slanted in such a way as to taint him.
It isn't a case of having to think they have a bias, it's a case of literally being told that bias is part of their policy.
It wasn't hard to spot, either. Their reportage during the Trump administration was absurd. You'd have headlines and first-paragraphs that were contradicted by the entire rest of the article; the headline and intro para made Trump look bad, the rest of the article begrudgingly attempted to convey the facts. It was utter nonsense.
Interesting. Do you have a link to the evidence of these instructions? I'd like to read them.
I think that label can be applied to most "news" sources these days.
To answer your question specifically though, it is because they report on events that other places don't, based on their particular bias.