He's popular for a lot of reasons. The biggest one IMO is the fact that he was one of the first to do things the way they are done now. Call it "The Modern Technique" or whatever you will, he was a pioneer in that field.
It's kind of similar to the way that a lot of guitar players revere the early
Led Zeppelin tunes. Face it, if any band got up on stage and tried to play like Zeppelin did back when their music was "groundbreaking", they'd be laughed at. But they were one of the first to do what they did, too.
Are there better pistol shots than Jeff Cooper? Are there better guitar player than Jimmy Page? Yes, to both questions. But, they will always be remembered as the "first" in their specialties, because regardless of what you think of Jeff Cooper's style of prose or his views on life (or, Jimmy Page's technique or his lifestyle), they were there "at the beginning".
ANM