My theory is that modern isekai is the next level of escapism. It's not just "some guy" fighting demons and getting the girl(s), it's "literally me!", a socially awkward loser, probably still in high school, who one day wakes up in magic fun land with super special magic powers (or a smartphone). The protagonist is a blank slate for the audience to project themselves onto. Meanwhile, the side cast is mostly just tropes, which means it's easy to switch between shows and not miss much. In particular, the women/girls in the show often fall into extremely played out tropes, so the audience can just pick their favorite, and because there's only very rarely progression in a romance subplot, any coupling is canon.

Also worth noting that "Isekai"/Portal Fantasy has been popular as a genre for quite a while (I want to say over a century), so this is less something new and more just optimizing the genre towards the type of viewers who are going to spend $30k on bodypillows.