I guess it's an age thing. My son's 18 and he hadn't heard of him either.
But he only retired from TV in 2000, that's not exactly a lifetime ago. He was just SO well-known. Raymond Glendenning, who was the David Coleman before David Coleman, retired when I was 7 but I'd certainly heard of him when I was 18.
I made a joke about Coleman doing general elections - according to Wikipedia (not always reliable), he did cover the 1959 GE for the BBC.