Trademark law. A more significant problem is that mainstream media publications often actively dislike each others' editorial stance, so the last thing they want to do is agree on a way of mutually endorsing each others' content. The UK, for example, has one right wing broadsheet, one left wing broadsheet that repeatedly attacks allegedly sloppy journalism at the right wing broadsheet and one notionally centrist broadsheet (run by a media empire that makes a lot of money from tabloids). None of them are likely to be receptive to the idea of certifying each other as being more reliable than other news sources.