I guess the short answer is that it's too remote for the internet generation's short-term memory and shorter-term interests.
But if Hollywood wants to recoup some of it's declining popularity, this looks like a great way to do it. I couldn't imagine major stars not climbing all over themselves to play the parts.
The guy's biography and "unsolved" murder just scream for a story plot that would resonate with modern events. You couldn't make up a character like his shady bodyman Edward F. Sands and the best black actors - even if the sexual connotations weren't removed
Feminists might have a problem with the evil controlling Hollywood mother who is the likeliest suspect for his murderer Charlotte Shelby. But I couldn't imagine a single A-list actress not wanting that part - especially if it's made the major role that it can and ought to be.