In the first place, there's a good reason for that. It's called an egregious MSM double standard. Southern accents are widely parodied by Hollywood and the MSM. Having a pronounced Southern accent will doom your career on TV or in Hollywood. But, for some reason, having a pronounced Northern accent is no handicap at all (Rosie O'Donnell, QED).
And, besides there are several counterexamples to your statement about there being no national newsmen with Southern accents. Charlie Rose, Bill Moyers and Dan Rather all have muted variations of Southern/Texas accents.
Do you really want someone who sounds like a Gomer Pile or a hardcore redneck giving you the daily national, international news & business reports?
I would rather focus on the news on not very distracting accents of network anchormen.