Advertising time costs money. The more money you can spend, the more time you have to argue your platform (and browbeat your opponent, but that's another discussion). Since we get the majority of our news from TV, that's where most of the money goes; stations can charge an arm and a leg for a thirty-second ad, and if you don't buy that time, well, you've lost the advantage to your opponent, who did buy it.
I didn't get a chance to see it, but a lot of reviewers -- including some who used to work on political campaigns -- have recommended the film The Ides of March as a reasonably acurate portrayal of the campaign process. It's fictionalized, but it might be worth taking a look.