I think it's because they have great hosts and good programming. I mean, lets be real, Larry King is old and boring. Anderson Cooper is a pretty boy, Keith Olbermann is a nut, and Rachel Maddow......ya, no thanks.
This article from the NYT, from this past March, highlights both MSNBC's and CNN's struggle to keep up with Fox: http://www.nytimes.com/2010/03/30/business/media/30cnn.html
This article from the NYT, from this past March, highlights both MSNBC's and CNN's struggle to keep up with Fox: http://www.nytimes.com/2010/03/30/business/media/30cnn.html