Writing in BSC review, Charles A. Tan explains why it's great to have an umbrella term like "speculative fiction":

The term speculative fiction holds a special place in my heart because it is able to accommodate all the other genres under its umbrella. What I mean by that is that fiction written under it can include elements of romance, mystery, horror, etc. In a certain way, the same can be said about the other genres....

There's also another advantage to such a vague term as speculative fiction. The only expectation one comes into reading it with is that anything can happen. Take horror, for example. When you read a short story that's classified as horror, you're already preparing yourself for the horror element to pop up. Compare this to reading a story simply labeled as speculative fiction, where you don't know whether it'll tickle your sense of wonder or frighten you (or both). While there's something to be said about the technical craft of writing a horror short story, sometimes it's better to not know that it's a horror piece in the first place for it to have a greater impact.

[BSC Review]