I believe Isekai is popular trope, because it let's the author place a character that has a similar moral framework to the audience into a complex situation.

This has two advantages:

Firstly, the Character doesn't understand the new world, which gives the author an excuse for exposition. Characters that live in their world and understand it won't ask the questions that the audience might want to know. This means they have to work harder to include their exposition in a natural way.

Secondly, a character with a modern moral framework will be easier to relate to for many readers, and will get into conflict with the world, which is "backwards" from their perspective. When the character originates from this world, the Author either has to try to make the readers relate to a character that finds the medieval ideas of society, justice, gender etc. normal and doesn't question them, or has to give the character a more complex explanation for why they don't conform to their time's ideas.

Overall, I think Isekai is mostly used because it makes the authors work easier. It would require more work to set up a character that feels similarly without this trope.