|"You're shinier than Shakespeare."|
I did promise that I'd give you some of my reasons that I believe these two genres are so popular. Fortunately, several of you briefly touched on my #1 reason, which is the fact that science fiction and fantasy can and almost always do encapsulate many, many issues. These issues can be societal, moral/ethical, spiritual, political, personal, mental, and emotional (the list most certainly does not end here). Since science fiction and fantasy stories often create their own worlds, it shouldn't be a surprise that characters in those worlds would deal with the same issues we do in our world, though with a smidgen more of lasers and swords than we encounter, I'm sure. Overall, science fiction and fantasy create worlds that mirror our own, worlds where we can safely ask "what if?" or "what is the reason we're here?" or "what does this all matter?"
As much as I adore reading Shakespeare's language, I have to admit that what the academic world considers to be literary stories only deal with so much, and that "so much" is often just one or two of the issues that I listed above. Hamlet is not a fair example; I think every issue I mentioned above was touched on in that play, so kudos to Shakespeare for that one. Hell, he even has a ghost in that play (supernatural issues should be added to my list...).
Let's look at Much Ado About Nothing instead. Predominantly, that work handles relational, societal, moral, and emotional issues; it is, after all, what I would call one of the earliest romantic comedies. You simply can't go into great depth about the other issues in this setting. Political issues? Sure, we see soldiers coming back from war and Don John wanting power, but that's about where the discussion ends. There's no description of whether or not the current government systems work, or why some of their bills are helpful, or why some of the leaders' tongues "outvenom all the worms of Nile" (Cymbeline). I realize that Shakespeare wrote A Midsummer Night's Dream, which is fantastical/mythological in nature, but I haven't read it yet, so that is why I am not discussing its handling of issues.
I believe that because science fiction and fantasy stories can cover so many issues in one book, world, or even universe, people simply cannot help but be drawn in by them. Sure, we all may benefit from reading a classic, or we might enjoy a heart-pumping action film on occasion, but we crave to know more. We crave to have our "what if?" question addressed, and thank goodness that characters such as Spock and Frodo can help guide us towards that end.