Marketing. Too focused on AI, followed by more than a decade of breathless hype for C++ and Java.

Nailed it. More generally: "non-technical cultural factors".

I would also add some minor incidental technical factors (e.g. early Lisp implementations were far less efficient than C implementation during the time when efficiency really mattered, and then cultural/marketing factors kept C as the dominant language far after the technical balance tilted in favor of Lisp).