Making it isekai means the story works better as escapism. Same reason Harry Potter is just some boring ordinary kid until he finds out his real parents were powerful wizards. Same reason Neo from The Matrix is just some run-of-the-mill office worker until he finds out he's actually The Chosen One.
Turning regular fantasy into isekai also creates a convenient excuse for other characters to drop an
infodump on the otherworldly protagonist, rather than letting the audience learn everything organically.