It's complicated, but there are likely some main contributing points.
1. Computing is extremely blinkered. More than in other professions, people in computing are either unaware of what has been done before. If they learn a little bit about what has come before, they look for reasons to be dismissive of it, so they can turn their attention away. They know a few things (languages, platforms, tools) and just live in that world.
2. At any given time, a small handful of things are popular. This changes over time. The amount of stuff we have produced in computing vastly outnumbers what is popular. It's like a game of musical chairs in a packed sports dome, where there are seven or eight chairs on the floor.
3. The field is still growing; there are probably more people who joined in the last 10-15 years, than those who joined the field before that. Almost every newcomer plunges into whatever is popular at the time, and will never look at anything else unless it is new and popular, which will happen at most some 3-4 times in their career before they are out.
Those are generalities. Then there are Lisp specific historic items.
Lisp specifically had a bit of a heyday in the 1970s and into the 80s. People developing Lisp systems were very ambitious and their work eventually demanded hardware that only big companies and well-funded institutions could afford. They did fantastic things, but Lisp did not scale down to the emerging single-chip microcomputer with a small memory (or not in that fantastic form). Typically, Lisp would have liked a few megabytes of RAM compared to tens or hundreds of kilobytes.
The microcomputer was something new and popular, bringing with it new people who had nothing but microcomputer experience. Most of them didn't know anything about Lisp other than reading about it in books or some magazines like Byte and Creative Computing, which is something that only a curious minority would engage in.
Eventually, consumer microcomputers became powerful enough to run Lisp well, but by that time, the people who remembered Lisp were vastly outnumbered by new people.
(Speaking of memory sizes, GNU's implementation of Lisp, even, GNU Emacs, was absolutely derided for its memory use, well into the 1990's. For instance, one joke interprets its name as an acronym for Eight Megabytes And Constantly Swapping (EMACS). Eight! Not Eight Hundred or Eighty. Eight megabytes is ridiculous today; the resident size of a Bash process can easily be that.)
Another problem with Lisp is academia, which has played a role in actively destroying interest in Lisp.
After the downturn in Lisp popularity, schools continued to teach Lisp dialects, but often badly, leaving students with a bad taste. They used scaled down dialects not suitable for software development work, like certain Scheme implementations, and gave students assignments that focused on doing things with recursion and lists, and other nonsense that is far removed from making a text editor or game or whatever.
This practice is still continuing. If you follow the [Lisp] tag on StackOverflow, you will notice that from time to time, students post Lisp homework questions. E.g. "we are required to write a recursive function that removes matching items from a list". The comments will say, there is a built-in function remove, why don't you use that. The student will reply, oh, is that right? But, in any case, we are only allowed to these five functions: cons, car, atom, ... and we can't use loops, only recursion.
Never in a programming course that used C, or Modula-2 or whatever have I had homework forbidding me to use any language statement type, operator, or library function! This is purely a Lisp teaching problem, and it leaves students with wrong ideas and impressions. They might misremember things and spread misinformation like "Lisp has only linked lists and nothing more, and everything must be done with recursion; it is useless".
1. Computing is extremely blinkered. More than in other professions, people in computing are either unaware of what has been done before. If they learn a little bit about what has come before, they look for reasons to be dismissive of it, so they can turn their attention away. They know a few things (languages, platforms, tools) and just live in that world.
2. At any given time, a small handful of things are popular. This changes over time. The amount of stuff we have produced in computing vastly outnumbers what is popular. It's like a game of musical chairs in a packed sports dome, where there are seven or eight chairs on the floor.
3. The field is still growing; there are probably more people who joined in the last 10-15 years, than those who joined the field before that. Almost every newcomer plunges into whatever is popular at the time, and will never look at anything else unless it is new and popular, which will happen at most some 3-4 times in their career before they are out.
Those are generalities. Then there are Lisp specific historic items.
Lisp specifically had a bit of a heyday in the 1970s and into the 80s. People developing Lisp systems were very ambitious and their work eventually demanded hardware that only big companies and well-funded institutions could afford. They did fantastic things, but Lisp did not scale down to the emerging single-chip microcomputer with a small memory (or not in that fantastic form). Typically, Lisp would have liked a few megabytes of RAM compared to tens or hundreds of kilobytes.
The microcomputer was something new and popular, bringing with it new people who had nothing but microcomputer experience. Most of them didn't know anything about Lisp other than reading about it in books or some magazines like Byte and Creative Computing, which is something that only a curious minority would engage in.
Eventually, consumer microcomputers became powerful enough to run Lisp well, but by that time, the people who remembered Lisp were vastly outnumbered by new people.
(Speaking of memory sizes, GNU's implementation of Lisp, even, GNU Emacs, was absolutely derided for its memory use, well into the 1990's. For instance, one joke interprets its name as an acronym for Eight Megabytes And Constantly Swapping (EMACS). Eight! Not Eight Hundred or Eighty. Eight megabytes is ridiculous today; the resident size of a Bash process can easily be that.)
Another problem with Lisp is academia, which has played a role in actively destroying interest in Lisp.
After the downturn in Lisp popularity, schools continued to teach Lisp dialects, but often badly, leaving students with a bad taste. They used scaled down dialects not suitable for software development work, like certain Scheme implementations, and gave students assignments that focused on doing things with recursion and lists, and other nonsense that is far removed from making a text editor or game or whatever.
This practice is still continuing. If you follow the [Lisp] tag on StackOverflow, you will notice that from time to time, students post Lisp homework questions. E.g. "we are required to write a recursive function that removes matching items from a list". The comments will say, there is a built-in function remove, why don't you use that. The student will reply, oh, is that right? But, in any case, we are only allowed to these five functions: cons, car, atom, ... and we can't use loops, only recursion.
Never in a programming course that used C, or Modula-2 or whatever have I had homework forbidding me to use any language statement type, operator, or library function! This is purely a Lisp teaching problem, and it leaves students with wrong ideas and impressions. They might misremember things and spread misinformation like "Lisp has only linked lists and nothing more, and everything must be done with recursion; it is useless".