Added later: information about why the border between post-Mexican-War Mexico ended up where it did, and what an alternate "lesser" Mexico would have been like if the border had ended up even further south - and what the American Civil War might have been like if Sonora, Coahuila, and Chihuahua had been absorbed and admitted as states.
Alternative histories have tended so far to be about events in European (or Western) history, because they're written mostly by European-descended people, and mostly by English-speakers at that.
But that's kind of obvious. So even forgetting the (so far) Western focus of what-if fiction, there are two clear patterns that betray some of our assumptions. And the first obvious pattern is: alternate histories are about violence transpiring differently. How many alternate histories involve policy decisions or inventions happening out of order? It's not usually: what if Newton and Leibniz had not developed calculus, or antibiotics had never been invented (or been invented in pre-Mongol-sack Baghdad)? No, the large majority of what we see are the effects of different outcomes of battles and wars. What if Hitler won, what if the North lost Gettysburg, what if Alexander or Jenghiz had not turned around at India and Austria. And this is depressing; because it means either that history is mostly determined by violence, or (even if it's not true) it means that we at least believe that history is mostly determined by violence.
Yes, there are a few stories about political decisions (what if the Ming had not called back the treasure fleets is a favorite) but it's really mostly about if people had killed and dominated each other in a different way.
You could also argue that what we see commercially is not an accurate reflection of our beliefs about history. It's the same answer for why in speculative fiction, dystopias far outnumber utopias. There's clear conflict and thus they're easier to write.
|The test at Sitio Trinidad, 1845, Jornada de Muerte, Nueva Mexico. The test was witnessed by governor Manuel Armijo and president Santa Ana, both of whom lost their vision as a result. (It seems both fission and high speed cameras were developed in nineteenth century Mexico but not dark glasses, go figure.) Despite this setback, the dreaded bomba santabarbara was smuggled in wagons and assembled in Austin and San Antonio and led to the end of the decade-long revuelta tejana and ultimately, the conquest of Alto México (previously the "United States".) Sure it did.|
The second pattern, or really observation that students of history can make about this sub-genre: even with battles won or lost differently, it's very hard to find believable changes that affect the outcomes. Sun Tzu was right, the battle usually is won or lost before it begins, and even if the tide on one battlefield had turned, the currents were running one direction or another. For example, so what if Lee won at Gettysburg? A big setback for the Union to be sure, but the Confederacy was screwed from the start in terms of their population and economy. To really get big changes, you have to make major shifts long before the obvious change - the kinds of changes that would have given the South a fighting chance would have had to begin many years before the actual war. Case in point: someone once asked me what might have happened if, in the Mexican War, the Mexican factions had unified against the U.S. as an external threat, and the U.S. lost its first major foreign war and gained no territory. (Or if the U.S. had decided to actually press its 54'40" claim.) A suddenly unified and organized 1840s Mexico is (unfortunately for patriotic Mexicans) only marginally easier to imagine than the first atomic bomb being engineered a hundred years early at Los Alamos by Mexican scientists. Such a story might actually make for some really interesting Latino steampunk fiction, but might as well also include unicorns.