I feel like the popularity comes from that people just like escapism.
When I think of stories that had characters being transported to a new world, I liked stuff like Wizard of Oz, Alice in Wonderland, and Little Nemo in Slumberland, which I guess are sort of like isekai from 100 years ago. I like them because of how weird and bizarre the worlds are, even to the point of being almost frightening and nightmare-ish at points.
And maybe some less old examples, Un Lun Dun by China Mieville and The Keys to the Kingdom series by Garth Nix.
I think some of me issues with modern isekai or at least the ones I tried to read that Tapas keep promoting, it that their fantasy worlds are so bland and boring. It just feels like an anime Ren Faire. And maybe it's a type of aesthetic that some people just like and they will eat up anything with it. But that isn't really the type of fantasy I like. And I feel like a lot of casual readers have become a bit sick of the Villain/Villianess/Tyrant tropes and the creepy power dynamics in the ones about romance.