Apple and Google now both require that you keep your app up to date with the new SDKs, even if your app works perfectly fine on the old version. As you upgrade to fix changes for that, it gets harder and hard to maintain support for older OSes. I'm not at all surprised that most app developers don't spend the time to ensure that their app continues to work on older devices, especially those that have a tiny marketshare now.

You can blame it on the developer for not supporting that, but it's in response to things that Apple and Google are doing, and I'd argue that they're really the ones at fault.