It was a long gradual process that didn't start with isekai; it has been decades of gradually accelerated industrial pressure to pander to the biggest spending otakus in Japan (and weebs in the west) because they bought the merch.

The pandering used to be pantyshots during an otherwise passable fantasy adventure focused on the adventure. Over time the focus increasingly became pandering to the audience, not just in horny/creepy stuff but also in "your video game skillz allow you to metagame and conquer this fantasy world" powerwanks.