Why?
The MSM doesn't like this boy(Cruz) so they have to keep him from being heard.
See Side-By-Side Shots of Sneaky CBS Editing of Ted Cruz Interview | NewsBusters
If they had asked a question and cut it because they didn't like the answer, that would be one thing, but the only issue here is Ted's inability to stay on topic.
They didn't ask "can you please eat up 2 minutes of air time with a rant about Obama?", they said "Will you run for president?". He even tried a second time to get him to answer the question.
When going on these shows, the deal isn't that you can just yell out anything that you feel like saying regardless of the topic at hand and expect them to disseminate it for you.