“Normal world” knowledge often does play a part, but more than that I think part of the draw of isekai stories is that the audience can more easily empathize with a character originally from our world trying to make sense of the fantasy they find themselves in than characters who are from the fantasy world and thus don’t find the setting particularly special.

It’s not like straight fantasy stories have died out, though. In the current anime season alone you have Sousou no Frieren, Goblin Slayer, S-Rank Musume, and Mahoutsukai no Yome—and no doubt several more—where the protagonists are native to their respective fantasy settings. That’s not even counting ones set in fantasy VRMMOs, which aren’t exactly isekai either.