Alternate history is a branch of sf with as much claim to validity as any other.
If you don't allow in alternate history, you shut out maybe half the what-if extrapolations available to the sf imagination. And, what's more, all future-based sf, with the passage of time, transmutes into alternate history anyway. Alternate histories explore worlds differing from our own by a small but conceivable change to the past. The most popular historical branch points tend to be decisive and dramatic events such as battles and wars, in which the alternatives generated by different outcomes are reasonably clear-cut. Some eras are visited more frequently than others.