Because in the 50’s and 60’s, high fantasy novels like *The Lord of the Rings, The Chronicles of Narnia* and *Earthsea* were incredibly popular, and many of their tropes and settings were so ingrained in the popular imagination (and codified in games by Dungeons and Dragons and similar tabletop games, the spiritual forerunners of essentially every fantasy or RPG game) that the genre was pretty solidified. Goblins and dragons and wizards tended to be associated squarely with castles and knights and such.
That being said, there is plenty of fantasy not set in that typical “high fantasy” sword-and-sorcery setting. Harry Potter is arguably the most popular fantasy series ever, and it’s set squarely in 1990’s Britain. Star Wars is science fantasy – a science fiction setting with fantasy tropes. An old wizard recruits a young farm boy and a lovable thief to rescue a princess from a fortress controlled by a black knight – replace wizard with “Jedi” and throw in some familiar names, but the tropes are there!
There’s modern fantasy and western fantasy, ancient fantasy, space fantasy, mythological fantasy, the list goes on and on. But the sort of heart of the genre, high fantasy (and its close relatives), really does owe its existence to those forerunners, and tends to sample heavily from their settings and tropes.
Well, they’re not; they’re set in an alternative timeline, mostly, which means it’s not really “medieval” in an easily-defined way. Heck, “medieval” is hard enough to define for real-life timelines, and any estimate of it lasts around 1000 years, give or take.
We call many fantasy novels “medieval” because they have swords and castles and people ride horses. But those things are not limited to the medieval period; fancy European castles didn’t exist for the first half of the medieval period, and stone forts of some kind have existed since antiquity, so they’re really not “medieval.” Swords existed for 4000 years before the medieval period and continue to exist to this day, so they’re not “medieval” either. Ditto horses.
So there’s things that you think of as “medieval” which aren’t medieval at all, and they show up in fantasy novels. I wouldn’t say that makes those fantasy novels set in medieval times.
And lots of fantasy novels don’t use that setting at all. Lord of the Rings and Song of Ice & Fire are pretty “medieval-ish,” as are the Arthurian legends of course, but most of the other very-popular fantasy novels are not – Harry Potter is contemporary, Alice in Wonderland is Victorian, 1001 Nights is indefinite and only maybe “medieval,” Narnia is mid-century.
Because it’s easier and carries a certain aesthetic that most consumers are already familiar with. It’s familiar, and stories like King Arthur and the Knight of the Roundtable, have been highly romanticized, so anything in that era already carries certain feelings of nostalgia, even among those who aren’t intimately knowledgeable of the stories.
I believe it has more to do with medieval being used too broadly to describe things. Do you just need swords and stone buildings to be medieval?
Beyond that I feel like it might be a confirmation bias. People expect fantasy to be medieval so that is what stands out, but as an avid fantasy reader (1-2 books a week) I find most share elements but are anything but medieval.
There are a few things that contribute to this particular aesthetic for fantasy fiction, but one significant thing to note is that there are plenty of fantasy stories set in other time periods, like the Dresden Files books which are set in the modern day, or RA by QNTM which features magic being discovered in the 1970’s. When an author departs from standard tropes it can be really interesting.
Really the largest reasons fantasy is often set in a medieval period is simply because that’s sort of what we’re used to; books like the Lord of the Rings set up a lot of our standard fantasy tropes, and it can be sort of jarring to see those tropes ignored. A lot of people just kind of expect fantasy to mean medieval, and divergence from that can hurt sales.
Another thing to note is that the “medieval period” displayed in a fantasy book can often cover a very wide range of potential time. We had people wearing armour and riding around on horseback shooting bows and arrows from something like 500-1600 AD. Many fantasy novels are set in a sort of generic mix-and-match time period that might include technology or social developments that come from all sorts of time (sort of like how the dinosaurs in Jurassic Park actually come from totally different periods across like 200 million years). So they aren’t really set at a specific time or level of technological development more than just kind of “old timey”.
Yet another factor is that, generally, if you want magic to be a big feature in your story, you need that magic to be *useful*. You want it to be something that, you know, matters to the plot. If you have high technology like an X-Ray machine, then you don’t really need a magic spell to identify illness. If you have cellphones with facetime, you don’t need scrying stones. Imagine if magic was discovered tommorow, but the only spell most people could learn to cast was a fireball. I don’t know about you, but I don’t actually need to know how to cast fireball, I can just buy a gun from walmart.
To add to other comments, a lot of it is just how society has chosen to define “fantasy” and the fact that the more the idea of “low technology and possibly magic = fantasy” is cemented in the public mind, the more people don’t consider fantasy that DOESN’T fit into that mold to be fantasy.
For example, a lot of fantasy is called sci-fi if it happens to happen in space, even if there’s very little other themes in the media that fit the science fiction genre.
For example, if you took Star Wars and put it in ANY other setting, it would very obviously be fantasy. But because you replaced swords with light-sabers and horses with x-wings and tie fighters and enemy fortresses with death stars and put it on space instead of on ground, a lot of people see it as sci-fi and not fantasy.
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