I can see how you could misunderstand. I think the value of appearing now, on the evenings preceding an actual election, in the middle of the primary season when people are actually starting to pay attention to the candidates is inestimable. Allowing people to become familiar with you now, when a decision about you needs to be made is good campaigning. An announcement on a popular show might have been a good move at the time, but it doesn't have the same impact on the electorate that an appearance now would have.
So you think the candidates just get to order the shows to let them appear?