As a few others have said, if you had a presenter with a very strong regional accent some people may not understand what they are saying. The much more neutral southern tone is better as most people can understand that.
Sorry but that's utter *******! You get people with strong accents in the South too as well. The people on the news don't sound like people from Kent or Essex to me. For newsreaders you do want people who can speak clearly, fair enough but intelligent people can always speak clearly with a regional dialect if they so want - Steph from BBC Breakfast for one.