Google releases Android, manufacturers & carriers do their modifications & put it on their phones (or not). Google then releases updates to Android, and some carriers on some models release updates sometimes.
Android 4.4 (KitKat) was released October 13, 2013, which means manufacturers & carriers have had 16 months to roll out updates on their phones and bring them the phones up to a more recent release.
I’ve had conversations with several people about why don’t carriers upgrade the OS on the phones, and the resounding answer has been, they want to sell new phones, not support he ones on the market. As a rule of thumb the carriers want you to buy a new phone at the latest every 18 months (wait isn’t your contract for 2 years?). It’s not like we don’t “buy” a phone from the carrier, enter into a 2 year agreement to keep paying them monthly, for a phone that they don’t ( it seems) plan to keep updated for more than what 6 months? It seems to me that we should have be having a conversation with the carriers about keeping the phones up to date for at least 2.5 years, beyond their launch.
Lets look at this from Google’s perspective, why should they push out updates when 60% of the users don’t have them anyways. One has to wonder if the users don’t have the updates because they don’t update anything, or because there are not updates available for them to put on their device.
Could Google have given some advanced warning? Probably, did they? I don’t know, I bought my Nexus from the play store, because I had an older phone where the carrier & the vendor did not release updates (beyond Android 2.3), well that and I could not remove apps I did not want.