Graham and Morris begin working on the dialect of Lisp as the two main dialects of the programming language, Common Lisp and Scheme, have not substantially changed since the 1980s. He says in 1985, a language was a specification but now a good, free implementation is required along with large libraries and regular updates.
Another thing has changed since 1985: Unix won. So there is a lot more agreement now about what you can expect from the [operating system]. Common Lisp and Scheme date from a time when languages had to be OS-neutral.