1) Different kinds of propaganda. Fox, CNN, MSNBC are businesses that reflect the biases of audience, owners, employees, etc. RT bias is support of the Kremlin.
2) Different levels of commitment. Fox, MSNBC, CNN are businesses pursuing their own interest. Their purpose is making money. RT was set up purely for propaganda. It's purpose is spreading propaganda. The "alternative news" is merely a method to deliver the propaganda.
Fox opposed Trump but then flipped to suit their interest. RT will never oppose Putin.
If Putin decided he wanted to genocide Jews (a totally unrealistic example just to illustrate the point), RT would come up with a reason why it's unavoidable, justified, and the US's fault anyway. If Trump came up with the same idea, even Fox news would probably oppose it.