The OP clearly is taking the position that fantasy and sci fi are different genres. Otherwise, there isn't much to this topic, is there?
But clearly there is some real world distinction in the popularity and appeal of the elves & wizards stuff vs the spaceships & aliens stuff, regardless of what label you want to slap on each. That distinction shows up in book stores and on TV, and somewhat less distinctively in movies. Hopefully this thread won't descend into a boring semantic quibble.
And I doubt that rebranding sci fi as "science fantasy" is going to do much to enhance its appeal. I'm sure book publishers would have done it by now if it were that easy.
The difference between sci-fi and fantasy is strictly aesthetic.
In terms of why one is more popular than another, yeah. People have been taught to believe aliens and robots are dorky and angsty teen vampires are cool, so they respond differently to the "stuff" in each genre. You can have the same story, but perceived differently, because of the "stuff."