The reason for that of course is that it's so trivial to embed it in a C program, that if you want a library you can just expose a C library in lua. luajit even makes this absurdly easy with its FFI api.
Right, but libraries consist of many other things, including ORM or testing suites. There aren't many great ORM libraries written in C, and you need the test suite to work in lua because you're testing lua code. In general, the world of lua is pretty small, probably smaller than languages like Clojure or OCaml.