Having great tools. Excellent documentation. Being friendly to new users.
Yes, it's also a systems language without a runtime. But that's not the novel part. You could write horrors in C++ that approximate ML even without language support. There are eldritch libraries where some kind of pattern matching is done via generic lambdas.
The main difference is developper UX. Good tools, good error messages, quality of life. The novelty is making ML not painful.