With Comcast finalizing its deal to buy 51% of NBC Universal from General Electric , skeptics are asking: Why would Comcast CEO Brian Roberts put his faith in Jeff Zucker, the NBCU chief who has dragged the NBC broadcast network from first to fourth place?

Because Jeff Zucker is one of the most determined, driven, ambitious, ingenious, competitive, compelling, resilient people you will ever meet.

Read “Life imitates TV,” a Fortune profile I wrote two years ago.

This is a guy who battled cancer twice. The first time, he was 31. Zucker, who is now 44, used to schedule his chemotherapy sessions at Memorial Sloan-Kettering Friday afternoons and then sleep all weekend, so he could work like a maniac at NBC starting on Monday morning.

His cancer recurred two years later. His wife, Caryn, was four months pregnant with their second child. Doctors removed 90% of his colon. Beating cancer, Zucker told me, “prepared me for almost anything.”

When Jack Welch was running GE, he was a Zucker fan. Dick Ebersol, the  influential head of NBC Universal Sports, has long been Zucker’s cheerleader–and his sway endures in this Comcast deal. Most critically, Jeff Immelt, GE’s current chief, has backed Zucker through good times and bad.

Two years ago, when I asked him about Zucker’s failure to prop up the NBC broadcast network, Immelt said: “I really don’t blame Jeff. I don’t accept it, but I don’t blame him.” He noted that Zucker, who has been at NBC for 23 years, inherited aging shows.

Indeed, NBC’s primetime profits, which peaked at $650 million in 2003, have dried up. But what counts more is that Zucker has impressively built NBCU’s cable networks, including CNBC, MSNBC, USA Network, and Bravo. Cable, with its dual revenue stream, is the far superior business model and where the big money is today.

Zucker’s decisiveness matters too. Immelt explained to me that he evaluates all his executives on five “growth traits”: inclusiveness, imagination/courage, expertise, external focus, and clear thinking/decisiveness. He rates his execs green, yellow, or red on each trait. Zucker’s “green”–the top–rating? Decisiveness. “He’s cocky. I kind of like that,” Immelt told me, noting that Zucker is “not afraid to make tough calls.”

Zucker’s weakness, in Immelt’s view? “He still has to work on external focus,” he said. Zucker has worked on expanding his vision. Now, with a new guy, Comcast’s Roberts, overseeing NBCU, he’ll have to work on it even more.