This occurs normally when your older device is no longer supported by the game.
However, for some high end Android devices, resolution might wrongfully cause a device to appear incompatible. In these cases it is likely that the device resolution has been lowered below a resolution that is required by the game.
If Google Play says "device not compatible", please attempt resetting your device resolution to Mid or High in device settings . Go to your Settings ->Display-> Screen Resolution, and change the resolution. This should take care of the issue.