Not too surprised. I know PHP has a reputation these days of being old and crufty but at the same time there hasn’t really been a killer replacement yet for the same use cases where PHP is/was used. React and Vue are all the rage for frontend work, but their paradigm is all about single page apps which is a bit limiting for something on the scale of Kbin. Other backend frameworks like Django tend to be fairly opinionated and lock you into developing in a certain way without providing a large enough benefit to make it worth it.
IDK maybe there are better frameworks that I just haven’t heard of. But whenever I go to start personal projects, the options seem to be Express, Flask, or PHP, all of which have their own tradeoffs. Personally I lean more towards Express or Flask but it’s not surprising to see people stick with PHP.
/r/place was super bandwidth intensive. They managed to turn each individual pixel change into several KB of data.