(1) It was very expensive to licence at a time where C was virtually free.
(2) It was a complicated language at a time where C was (superficially) simple. This made it harder to port to other platforms, harder to learn initially, etc.
(3) All major operating systems for the PC and Mac happened to be written in C.
Ada had virtually nothing going for it except being an amazingly well-designed language. But excellence is not sufficient for adoption, as we have seen repeatedly throughout history.
Today? Virtually nothing stops you from using Ada. For lower level code, it's hands-down my favourite. Picking up Ada taught me a lot about programming, despite my experience with many other languages. There's something about its design that just clarifies concepts.