Why are most isekai happening in a medieval world?

I think you might have landed in a category of Isekai novels where Medieval influence is more prevalent. As a avid reader of Isekai's myself, I can confirm that there are also plenty of Isekai's available that do not draw from the medieval ages, but focus more on other eras or even on far more futuristic worlds.

However, as you validly point out, medieval influence can be found in in a lot of media, including video games, board games, manga, and of course novels. However, more often than not, those are not truly set in the medieval time period. They just tend to build upon the concepts of the medieval age, for example feudalism, and build their society around these concepts, all the while introducing their own mechanics such as fantasy races, magic, geo/demo-graphics etc.

This way of story writing is quite common in the the fantasy genre, and even more often so in the sub genre high-fantasy. Which, quite a lot of the Isekai's would fall under.

One of the reasons this happens is because it eases the burden on the writer but also the reader.

Rather than create an entirely different world that readers will recognize as alien, it's often simpler to revert to more primitive times, and simply add in magic when its existence doesn't require you to create a dozen centuries of magitech. It's a lot of work for the writer, and it may confuse or alienate the reader. - Jerenda @ Worldbuilding.SE

Additionally, the introduction of factors such as magic might also reduces the need for a society to evolve beyond the Medieval era, or at least, not in the same way as our society did. Why would a magic world be stuck in medieval times?

Most of the Isekai's that I personally encounter also tend to follow the High-Fantasy medieval settings. But occasionally I see a twist of Heroic Fantasy, Low Fantasy, Dark Fantasy and sometimes even Grimdark Isekai's.

To name a few:

• Youjo senki set in a world war 1/2 era (reincarnated as a war prodigy)
• Mezametara saikyou sobi set in a sci-fi space setting (reincarnated in a spaceship)
• Musou Kouro ~ Tensei Shite, Uchuu Senkan no AI ni Narimashita set in a sci-fi space setting (reincarnated as an spaceship AI)
• A returners magic should be special set in a relatively modern magical world (reincarnated/returned in their own world/timeline)
• Moshi fanren a modern day world struck by a zombie apocalypse from space (reincarnated/returned in their own world/timeline)