It's been quite a while since I dipped my hand in Android development, but from what I recall when you made an Android project, you had to set the API "Level" -

It's been quite a while since I dipped my hand in Android development, but from what I recall when you made an Android project, you had to set the API "Level" - which changes every new version of Android. I assume Google is looking at that value, comparing it to the current running API and blocking installation because the API the app is using is too old, with new permissions conventions that are built into the newer API versions running on more modern phones. I believe they also limit(ed) installation of apps made for tablets on phone screen devices (and vice versa) through the devices' DPI settings. I believe you can easily sideload these apps into Android and have them run mostly flawlessly, but Google Play Services blocks install probably to prevent people from installing old/possibly incompatible apps on their platform.