Going to echo it's not an iOS 12 things vs more of a display size thing. Saw same with iPads when they came out, took a couple of years for many developers to adjust their screens for the new device screen: lots of apps, including a number of Google apps, were ignored.
As mentioned, from a basic os standpoint, nothing really big between 11 and 12.
Comes down to developer wanting to put in the effort to adapt the app. And if their old app is not making money for them, might come down to them not thinking worth the effort, even if not that big a move for them.
And add: as mentioned, app might have been abandoned some time ago, so, that variable is added to the equation (guessing old games fall into this catagory).