This may be the case in the US, but the BBC seems to be encouraging the use of regional accents by its newsreaders.

US broadcasters are probably not encouraged to keep their accents because many Americans find certain accents very annoying (New York, Boston and Southern specifically.) I'm sure if the newscasters had British or Australian accents they would not be encouraged to lose them because so many Americans love them. The Crocodile Hunter is incredibly popular here. Much of that probably has to do with his accent. A local radio station recently hired a female British DJ, obviously partially because of her accent.