The goal of the media until the election is first to milk the candidates for money

Which basically means that some candidates must be kept within remotely plausible striking range of the front runner or else there is no reason for any of them to spend money on ads.

The second goal of the media of course is to make sure that the established system is maintained as nearly as possible to the status quo, that is where their wealth and power flowered and from whence it flows now, radical change is bad because it will upset the power and wealth structure.

Note here that I'm not necessarily talking about some grand conspiracy but rather a lot of different self interests coinciding in similar behavior.

The media has no interest in letting any candidate get too far ahead, no big ad buys in that scenario, no they want a horse race but they want the politician most committed to the status quo to be the eventual winner of that horse race.