Because 40 years ago people were convinced it would be popular one day. And 30 years there was roughly the same percentage of highly convinced, vocal people who thought so. And 20 years ago same thing. 10 years ago ditto.
I mean, what is going to take? 10 more years? 20? At what do you conclude that a language really isn't useful for general purpose programming? (not to say it isn't brilliant and useful for specific purposes, learning, etc.)
You seem to define the reason as purely historical. Kinda like how Linux was going to take over the desktop "any year now" and it hasn't. Yet Linux is everywhere people don't think it is. It's on most of the world's smartphones, for instance but people still consider it "failed".
I think Lisp is the same: it's everywhere you don't think it is because it's opinionless and languages are defined by their community (and community is held together by the shared values and opinions). Look how many languages are rapidly adopting functional programming idioms. They're becoming Lisp. Eventually you start seeing the value of functional idioms in Blub language. Then you go and look at where it came from and notice, "Hey this is the real thing". Then you get real yourself. (This has been my experience).
Let me lay the raw truth out for any lisp fans. It will never be popular or widely adopted. Ever.
It has been around for half a century. At no time was it ever widely adopted. It was always just pushed by a small group of fanboys.
That said, I love it and have learned a ton from as have many others.