Almost certainly familiarity and nostalgia (Score 1)

Fantasy is always going to be more popular than sci-fi because of the implied sense of things past and the past is always a golden age in the human psyche. Sci-fi tends to imply the future which is just too difficult for most people to even visualize. Successful fantasy settings are all pretty much the same medieval time frame and everyone knows all about that. Sci-fi settings can be anything and good luck getting a group of people to agree on anything.