This is not something that is clearly true. In other words, it is not at all clear that the media should be required to provide equal coverage to all political parties and all candidates. There are many reasons why one could argue that this is a very bad idea. For example, you could argue that it violates the First Amendment. It is hard to reconcile freedom of the press with a situation in which the government tells a newspaper whom to cover. It would also be somewhat ridiculous as many elections attract large numbers of fringe candidates with absolutely no chance of winning.
That said, if you must argue that the media should be required to do this, there is at least one argument to be made. This is the argument that we cannot have a true democracy when some voices are shut out by the media. From this perspective, the media prevents us from hearing about a number of candidates whose ideas we might agree with. This means that we do not get to fully exercise our right to choose our leaders. From this perspective, the media should have to inform us equally about all candidates so we can have a chance to fully evaluate all of them. This would be more democratic.