In fact there's probably no single reason accountable for Lisp's lack of popularity.
There is this one: simple statistics. People have created an astonishing number of programming languages, of which only a vanishing minority are popular. If you pick a language randomly out of all of the ones that have been ever created, you will with a very high probability close to 1 land on something that is not used at all.
Given the age of the first versions of Lisp, it is astonishing that the descendants are still here and that there is a lot of resemblance.