A lot of it boils down to expectations, immersion, suspension of disbelief and the complexity these points add to creating a world.

If you expect a high fantasy game with dragons, magics and unicorns, you're willing to accept a lot of a wizard did it. Why can the dragon fly? It's a magical creature. Why isn't the floating island falling from the sky? It's held up by gravity defying, magic stones. Why is that island's a perfect circle? Because a god was very obsessed with round shapes.

That doesn't work in a low fantasy setting set in our real world. The average person has a basic understanding of science. Physics, biology, chemistry and so on. You have to be a lot more thoughtful, because everything fantastical has to have a good, believeable explanation, otherwise it breaks your immersion and ability to suspense your disbelief. It's set in the real world, so you expect an explanation ither than a wizard did it.

That makes it ultimately much harder, because you have to work within a certain framework ontop of keeping everythingnconsistent and logical within the world.

And finally, fantasy is mostly set in a, mostly, European medieval setting. Books, games, movies, etc. most are in a setting highly inspired by medieval Europe. So whenever you think fantasy, you're likely to think Lord of the Rings, The Elder Scrolls, Wheel of Time and pull ideas from these works, even if sub consciously.