Most people are using their personal devices for entertainment these days. And most of these devices use Bluetooth or USB-C now anyway.

So why go through the expense of updating the old planes and headphones? They chose 2pin early on, so that became the standard.

Most planes have a 30ish year service life. So the older planes now are from the early 90s or so. So you should be starting to see more 3.5mm, as it was pretty well established as a standard for personal music players by then. The walkman had long since used it, for example.

But updating electronics for planes is expensive and time consuming due to regulations. So there will be some lag in terms of newer planes rolling out with it. And you're probably flying more older planes than newer ones, in general.