...I wouldn't exactly put RT into the category of being much better than CNN and the like. Breitbart is also seemingly more accurate and honest because of confirmation bias - they are more selective in their reporting than mainstream news, thus they report on things that they have usually investigated and/or are confident in more often. That said, they're just as susceptible to folly and hackery as other decently large "alternative media" outlets.
As for why they tend to be more accurate and honest than the main stream media news in general? It's basically because their business model requires it. Fox, CNN, MSNBC, ect. are all insanely well funded and have a large viewerbase and reputation (what's really puzzling is how CNN still has any of either of those two after the last ~5 years or so) and can shit out what they want with few consequences. If CNN does ill-researched hit pieces on current events or people, drug companies and car companies are still going to pay shitlords of cash to advertise during their programming blocks regardless of how it's received. In fact, the more "clickbaity" that CNN gets and the more people that watch them solely out of anger (a surprising amount of the dwindling and now current TheYoungTurks viewers are actually people that hate-watch them), the better it is for them to sell advertising space. So CNN being shit and controversial means higher ad revenues.
Fox, for much the same reason, can pull the same shit. It's a news media hugbox for some, a hate-watch media outlet for the left, and it's currently leading in the ratings for American news media outlets. Hence they can do what they want with little - if any - repercussions.
TL;DR: it's because the American media is shit.