Is it political, or is it a business decision?
The news media thrives on controversy and drama. If all they could report to us was "Everything's great here in Iraq," they'd probably begin losing their audience. But if they can put us on the edge of our seat, we'll keep tuning in.
They need viewership to survive, so they spin things in a way that keeps us coming back. If there's an effort to supress positive news, I'd guess it could be traced to business decisions rather than political inclination. But even if there is some anti-Bush sentiment in the media, that too can be explained by the requirements of the industry: a horserace is more exciting than a cakewalk.