The other genres of speculative fiction are entirely their own things. Take alternative history, that can be science fiction or fantasy or even horror, but things don't tend to trend towards alt hist, it either is or isn't. Compared to scifi, fantasy, and horror wherein things trend towards them, such as LoTR or Foundation. Same can go for dystopian fiction, dystopian fiction is its own thing, can be any other combination of genres, but within speculative fiction it's themes are too niche and your won't see works trend towards it in a gradual manner, either the work is dystopic, or isn't. Scifi, fantasy, and horror are much, much broader genres that encompass a number of themes and can combine with each other or any other genre, so lots of stories can easily fall within that triangle, and depending on what is going on in it you'll see a work float between it. >that shit take on themes
Death and insanity are two major themes of horror, but if your story places more thematic focus upon other facets such as say war itself, with death and insanity being secondary to it, it's not going to trend to horror unless it keeps the tone there as well. You'd have to be absolutely retarded to not see how shit like advancements in technology is typically associated with scifi, but unless there is major focus upon that theme in the story it's usually ignored. Like fantasy settings may have guns as the up and coming new thing but because it doesn't pay much heed the theme of new technology is of little importance to no importance when prescribing a genre name to it.