Unlike REST, HTTP is a well-defined standard. Hypermedia APIs, on the other hand, are an active area of research, so it makes perfect sense for them not to be

Unlike REST, HTTP is a well-defined standard. Hypermedia APIs, on the other hand, are an active area of research, so it makes perfect sense for them not to be standardized yet. And once a specific standard does emerge, it will have a different name too, of course.