Having recently watched the mini-series of Childhood's End I went back and re-read the story. It's one of my favourite SF novels, as in when I had to downsize my collection it was one that I kept because I felt I would want to re-read it.
Clearly the reader demographic has changed over the last 60 plus years since Childhood's End was written, which is a thing, because to maximize sales (readers) one has to appeal to as wide an audience as possible, and classic SF really doesn't do this. This may be a factor in why written SF is a small market.
For example, Childhood's End. Who is the protagonist? Who is the antagonist? Where is the character development? It's arguably one of the classic novels, yet today it would be a hard sell.
NB: Just to be clear, love Childhood's End, and Arthur C. Clarke remains one of my all time favourite writers.