There are many explanations for Bertrand Russell Berns' relative obscurity. The subject of Bang! The Bert Berns Story flopped as a performer, and so turned to songwriting and producing. He sometimes composed under aliases such as Bert Russell and Russell Byrd. And several of his tunes became associated with their performers, who were widely assumed to have written them.
Also, Berns died young, succumbing to the long-term effects of childhood rheumatic fever at 38. It was 1967, and rock 'n' roll was just beginning to be chronicled by sympathetic observers.