I think the problem there is that the contravention of consensus reality often isn't the point of the story, and usually isn't noticed by anyone within the story. In a mystery, at least some of the characters are aware that the puzzle exists, and if it is solved* it is solved by characters in the story, not by Deus Ex Machina. [Random person not previously seen in the story showing up on the next-to-last-page and confessing isn't considered proper, nor are divine voices telling the detective "arrest the guy in apartment 4, he did it and if you tell him you know he'll confess and show you the knife.") Similarly, the people the romance is about are aware of each other and of their feelings, at least by the end of the story.

In most sff, the characters aren't aware of consensus reality being contravened. Their world is the real world. It may have alien spaceships, or a different history from ours, or magic that works. But the story isn't about history having changed, and if it's about the spaceships showing up or the magic coming back, the reaction is "this is weird|scary|wonderful" not "this can't be the real world, because in the real world magic doesn't work, and Nixon lost in 1960."

So, yes, sff are stories set in alternate realities, many of which fall into identifiable patterns, but they aren't stories in which reality changes. [Not usually, and that tends to get metafictional, as with R.A.W. Wilson's Schrodinger's Cat or Matt Ruff's Fool on the Hill.

*It's arguable whether, say, McBain's He Who Hesitates is a mystery, despite being clearly a novel about a crime.