Possibly because they grew up with the generic shonen stuff, so that has a special place in their heart, as opposed to isekais, which the trend is pretty new.
In a few years when the new generation of gatekeepers show up, they're going to defend all the generic isekai shows. It happens in other entertainment genres, especially in music and video games, where some of the stuff that's praised today were hated back then. A lot of the games and bands that were popular and new 10-15 years ago were hated by gatekeepers are now being praised by the new generation of gatekeepers, because they grew up with that stuff.