"We tend to think of magic as part of history and technology as part of the future, but consider this: magic as projection of human desire, and technology as a projection of human will."
That got me thinking about the casual truism that fantasy is about the past, while science fiction is about the future. While I don't consider this a rigorously defensible proposition, this is sufficiently true that we want to sub-brand future fantasy as "science fantasy," for example, while past sf becomes "steampunk" or "retro."
I can draw other distinctions between the two genres -- for example my prior assertion that high fantasy virtually requires a moral axis, while Silver Age SF avoids the same --but I wonder if the temporal facing is the underlying appeal. I also find a lot more ambiguity in SF than in fantasy, which somewhat parallels the moral axis idea.
No real point here, just noodling. Feel free to noodle along in comments.