How did "other world" or "isekai" stories gain so much popularity and where do they have their roots?

Question

There are and have been so many stories of that type being released, especially in the light novel or manga section, and I noticed that these stories often follow a similar pattern and have a similar setting:

• the "other world" mostly has a medieval setting and is often constructed as an RPG with a leveling system.
• That world normally contains different fictional species (humans, elves, demons etc.) who are usually capable of using magic.

If the protagonist isn't born into that fictional world he's either being transported there or reincarnated. And these two ways also mostly follow a similar pattern, aside from the fact that the original world is Japan in the 21st century:

• if he's being reincarnated he'd usually died in his original world, normally due to an accident with a truck.
• If he's being transported there it's often due to a summoning by a royal family. They want the summoned "heroes" (it's often not only the protagonist who's been summoned) to help them to defeat the demon king who's invading the country or at least is a threat to the people. If the protagonist wasn't summoned by anyone he often becomes an adventurer.

Where did those "clichés" come from and what makes these types of stories so popular?

Answer 1

Isekai: The genre that took over modern anime is a video by Gigguk that, while amusing, also covers a lot of the actual history. Essentially, the idea of being pulled into another world is at least as old as "Alice in Wonderland" (although you could probably find some proto-isekai in works like Dante's Inferno or even folk tales of the land of Faerie). What's more interesting is that there was a lot of isekai anime around the 80s and 90s, but it was mostly aimed at a female audience; it's only more recently that it flipped to being targeted at the male audience.

Isekai is essentially based in wish-fulfilment, or the desire to insert oneself into a story, so it's a combination of that with the popularity of JRPGs (since that's what most modern isekai are based on). The rise of self-published light novels and manga has then resulted in a lot of source works with self inserts and isekai concepts, which then become popular and get made into anime.

After the genre starts to become popular and develops its tropes, you then start getting the usual response of works that deconstruct and otherwise play with those tropes - things like "What would it actually be like to be pulled into a video game world where you actually die and then respawn at the last save point?" (Re:Zero), or "What would a typical video game player actually be like in a video game world?" (KonoSuba).