Lack of care mostly, especially on the Java edition it has gone downhill in performance rapidly ever since Microsoft has owned the place.

I love how they make Bedrock edition for "performance" and then completely mess up on doing anything with it.

I’m not sure how this was handled on different platforms, but a recent update actually doubled the size of worlds by making them way deeper. It doesn’t seem like they did squat to optimize it in the process, playing split screen with the kiddo on ps4 gets pretty laggy. They made cave generation way more complex too, so maybe that taxes it more when entering a new area but I cant really tell

It ran while simulating a distance of one chunk at a time. Computers could simulate much more because the game was written to let software define just how far from the user to render. Phones rendered hardly anything which let them at least play the game.