It seems like the author is conflating two things, Lisp not being popular and there being many Lisps. As others have stated, the reason there are many Lisps is

It seems like the author is conflating two things, Lisp not being popular and there being many Lisps. As others have stated, the reason there are many Lisps is because it is very easy to make a Lisp. You can check on github for Lisp implementations and find very many. The reason why it isn't popular is something I don't really want to get into. But, I will say that modern languages are transitioning much closer to Lisp than they are backtracking to languages like C.