That’s because as a candidate for political office the network would have to provide equal time to the competitor(s). He should have known/expected that.
Same thing happened to Mike Huckabee. He should have expected this and probably did. It’s more fun playing the victim, though.
Worst. It would be considered an “in kind contribution” if they allowed him to stay on the air and talk about his campaign. His air time would be millions of dollar of free political advertisement.