Google stated as much on a support page, noting that because the phone was released two years ago, it doesn't meet the 18-month window where devices normally receive updates.
There's also word over at Engadget Spanish the phone won't get the upgrade because it carries a chip made by Texas Instruments, a company that no longer produces mobile silicon. Therefore, it wouldn't be able to provide support for the new operating system. Google hasn't confirmed this is the reason behind the decision, but it seems likely.
It's lame to be sure, but then again, it's probably time for you to update your phone after two years anyway.
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We'll probably even see the sweet new system on budget phones soon afterwards since the OS is compatible with as little as 512MB of RAM.
Google notes on its developer site that, "KitKat streamlines every major component to reduce memory use" for a broader range of devices, meaning the whole OS has been condensed, along with memory used by Google services like Chrome and YouTube. More memory efficient apps can also be created.
That KitKat OS
4.4 KitKat is seeing a lengthy range of improvements; the interface is much cleaner and boasts changes like full-screen album art during music playback to better search options like new voice command options using "OK Google."
According to Gizmodo, the OK Google feature requires heftier hardware like Snapdragon 800 so it's only out on the Nexus 5, but don't count out other Android smartphones yet since many are likely more than capable.