Many OEMs just don't care. I recently asked ZTE about my phone, which I bought in January.
Their answer was, It's a device we designed in 2013, we no longer care about it. So I'm stuck with Android 4.1. Hopefully this phone will last long enough for something that actually meets my needs comes on the market.
That or maybe I'll just wean myself off the need for a mobile phone and the mobile carriers/phone makers/etc can do without my business.