Larry Cohen was hailed by critics as a talented, prolific writer, but why didn't any of the series he had go too far beyond one or two seasons?
The reasons vary, depending on what show you're talking about. As we previously discussed, Cohen's central theme to virtually all of his works was having a lone protagonist on a mission of some kind. In Branded, it was a disgraced Army soldier seeking redemption. In The Invaders, you had a man trying to prove to a disbelieving world that aliens had arrived on Earth, seeking to take it over.
Then, there's Coronet Blue, which was actually produced in 1965, but CBS was forced to sit on it for 2 years before it could finally hit the air. By the time it did, in May 1967, any hope for renewal was gone, since star Frank Converse had moved on and signed with David Susskind's Talent Associates to star in N. Y. P. D. for ABC that fall. Converse played Michael Alden, an amnesiac who was really a Russian spy sent to the US. However, he grew to like it here, and decided to defect. His superiors didn't like it, and decided to kill him. The open has Alden pulling himself out of the river, the drugs his former comrades put into him having wiped his memory.
The memorable title song was performed by Lenny Welch, who was asked to emulate, if not imitate, the stylings of fellow singer Johnny Rivers, who'd scored a chart hit with "Secret Agent Man". Ironically, Secret Agent, aka Danger Man, was also airing on CBS.