Have you ever heard of that psychological study that claims the working memory of your average human is seven things, plus or minus two? What this means is that most people can remember, on average, seven "chunks" of information.
So what does this have to do with Lisp?
Many programming languages can be read (and thus programmed) in a very orderly fashion: top down and left to right.
Lisp programs don't seem to follow this convention. Instead, you sometimes finding yourself reading right to left (because of returning function values) as well as left to right, and also bottom-up (again, function returns) instead of just top-down. Lisp programs feel a lot more interconnected to me than many other languages. I always find myself looking for breakpoints where I know the state of the program and can take a breather. Lisp doesn't give you many of these.
Tracing through a Lisp program feels like following an M.C. Escher picture. Things go everywhere and loop back again inexplicably, which is tough when your goal is to understand it.
Making matters worst, the Lisp culture appears perfectly content with this complex style of coding. You don't really notice this until you examine other programming cultures, such as Forth, which have the same potential for abuse, but try to avoid it by explicitly encouraging a very concise style of programming. Forth programmers shoot for only 2-3 lines of code per function (word in Forth speak). I don't see this same cultural value is Lisp where verbose, multi-stage functions appear perfectly normal.
Which brings us back to that working memory limit I mentioned earlier. All this complexity increases the amount of program state that programmers must keep in working memory in order to understand their programs. Think about that!
I bet that if you tested many of the programmers who promote Lisp, you would probably find that they have excellent memories or have discovered some memory scheme for conceptualizing their programs. Maybe they've just learned to ignore the parts of a function they aren't working on. I don't know. I do read comments by other programmers which mention that memory is a key feature they look for in other programmers.
Here's this blogs thesis: Requiring programmers to keep track of more than five or six non-linear details at the same time will render a programming language more difficult than its competitors and lead to reduced usage.
Programming languages should be designed with this limit in mind and programming cultures should consciously promote it.