Greg Cox wrote:
Well, I can't remember who coined the term, or every critical essay I read thirty-plus years ago, but I suspect the idea was to come up with a more inclusive label that acknowledged that there was more to "science fiction" than just the whole John W. Campbell Astonishing/Analog school of nuts-and-bolts sf. There was also Bradbury and Edgar Rice Burroughs and Sturgeon and Ellison and Spinrad and Silverberg and Delany and LeGuin and Russ and so on, whose work was arguably more about the "fiction" than the "science."
I know that Ellison favored the term at least as early as the first Dangerous Visions
(1968), if not before, and it's believed that the term may have been coined by Heinlein in the late 1940s.