It's a sad state of affairs, I'm in the same boat. What made the situation worse is that not so long ago Amazon actually sent a message to my HDX advising the fire OS 5 update will come to my tablet, only to send an email stating it was an error, maybe an apology was in there too. Looking on XDA at the time, I wasn't the only one who received the message.
Idiots.
As for why it isn't coming? It's not planned obsolescence, as Amazon makes little to no money on hardware and is all about getting you into there ecosystem. It's silly really, as the hw is more then capable.
I reckon it's due to the failure of the Fire phone, I read a lot of people were let go, engineers, software devs etc, due to the failure of the fire phone. Which coincidentally, is also powered by the snapdragon 800, like the HDX.
You what makes it worse? The generation of tablets that came after the HDX received the update. It's a little infuriating.
It's still a pretty good tablet for media consumption, web browsing, shopping, the kids mode and NOT much else, so the OS version is bearable for now.
Regarding the possibility of forcing the update on the HDX, a quick glance through XDA doesn't bring up much.
There is a thead about the update, and a post about a customer service rep stating the Fire OS 5 will come to the HDX, but other posters have pointed out that it's a rep confirming this, not a dev:
http://forum.xda-developers.com/kindle-fire-hdx/help/fire-os-5-support-3rd-gen-kindles-t3204316/page19
