Actually, "library" is probably the wrong word for reused Lisp code. A library is a public thing, an institution---and all that goes with it: slow to change, must be easy to understand, etc.

Instead of libraries, Lisp hackers have private collections. Like a messy desk, they are often supremely useful once you get the hang of it, but are inscrutable to others. Not exactly the best characteristics to get popular on github.

But also its not that its great for writing libraries but that its great for writing libraries that extend the language.