'Cause it isn't really genre. It's two techniques that are both alternate history. The first is to introduce a point of divergence and extrapolate from there. Time travel or the "many-worlds" hypothesis in quantum theory is the commonly invoked technobabble. The other is have a magical (or otherwise different) world and parallel real-world history.
The fun of the latter game is to see how you can smuggle in all the magic or what-have-you without changing history and make it plausible. Poul Anderson's A Midsummer's Tempest, for instance, had the English Civil War happen on time, even though William Shakespeare is the Great Historian in that world. (And why would that make a difference? Because in Julius Caesar, a clock chimed. Chiming clocks were a medieval invention. Anderson concluded that it was always more technologically advanced and the Industrial Revolution is in full swing at the time.) Or Patricia C. Wrede's Mairelon the Magician, where we have the Royal Society of Wizards but the Napoleonic Wars happened on cue. "Magic, Incorporated" is modern America with replacements for technology. Leviathan is steampunk and all its technobabble is science-based, but it falls under this; WWI looks like to happen on cue.
The fun of the former game, however, is the extrapolation. Except that many writers extrapolate the results of the change, and then have that go on for decades or centuries. G. K. Chesterton once observed of predicting history that if you see a river meandering all over the landscape, you might have difficulty predicting where it would end up -- but you would be safe betting that it would not abruptly start moving in a perfectly straight line. That is what I see again and again. In Bring the Jubilee, the North is still mired in economic depression caused by, and decades after, its loss in the American Civil War. (Nevermind that winning Gettysburg could not have won them the war; Antietam, maybe, but they had lost Vicksburg just about the same time.) In Poul Anderson's Delenda Est, the path is a little more complicated: Carthage defeated Rome, Rome did not help defend the Jews during the time of the Maccabees, Judaism was wiped out, Christianity did not result, science did not result. But -- the mindset in what would have been the time travelers' own time was exactly what it was in the time of the Punic Wars. Randall Garret's Lord Darcy works -- well, much as I like them as mysteries, they diverge when Richard the Lion-hearted survived, and since then, it's been all Plantagets on the throne with no upheavals; the only historical event cited after that was when they elected one of the Holy Roman Empire.
So -- I like the chiefly fantasy type better because the world-building history is much more lively and interesting, where the chiefly SF type is flat and improbable.