"I find it amazing that more people don't still know about him."
Here is a singer who died 30 years ago and whose popularity peaked 50 years ago, and you find it amazing that two generations later he is relatively unknown? (I assume you mean among people born since his death.)
I don't know why it is amazing to you.
Eventually the same will happenm to Sinatra, bearing in mind he died less than 10 years ago and has had 4 albums of new material released since his death, the latest only last year.
How much do you know of Stephen Foster, who was, in terms of popularity in his own time, possibly the most successful American pop song-writer? How about Rudy Vallee, who was essentially the model "pop idol" for both 'Der Bingel' and the 'Chairman of the Board'?