This may sound ageist, but it may be because you're just 18 and haven't lived enough yet. Most people don't upgrade android. Most people can't upgrade Android.
Per http://developer.android.com/about/dashboards/index.html about 33% of devices are Android 5.0 or higher. More than 50% of android users are using 4.1-4.3, and that probably won't shift massively in the near future.
I too have a nexus, but AFAICT, I'll never get anything beyond 4.4 (unsure right now, haven't even turned it on in a while).
Perhaps, maybe, in a few years, policies will change enough to say "if you use this brand, you have to adhere to these policies" and maintenance and upgradability will be a required policy. But... the effects of that will be years in to the future - there's no real change in the near term.
Google seemed to intentionally ignore that aspect of licensing, and... somehow we're all collectively dismayed that hardware manufacturers chose their own short term profits over the impact to the Android brand?
Well just to chip in, the last stable Cyanogenmod version for my device is based on Android 4.4. There are some nightlies, but never having updated to those (expecting a stable at some point) and being quite happy with my current OS, I'd rather not update.
(disclosure: I work there, however not on the product side)
One has to create an account before one can use harvest, which is a huge turn-off for me because I don't want to create an account before I even know whether your app is the app I'm looking for. I would probably have skipped your application while searching for a solution because of this.