It didn't sell people didn't want it, it was way to big for most users, and way way too expensive (~400$ for the 3G version). People aren't going to pay iPad prices for an ereader of the same size, weight and these days almost the same battery life....
I love my kindle voyage but given the choice between a Kindle DX and an iPad air which costs almost the same most people would get an iPad and so would I.
Browsing is needed for manuals and similar, those are also where you need a larger screen for PDF files.
(Literature is totally ok on a small screen with slower refresh.)
The Kindle battery last for 1-2 months (book reading, it only consumes energy if you turn the page -> e-ink display!).
The iPad battery last for 1-7 days, depening what you do with it and how bright you set the IPS display.
Many own both an iPad as well as a Kindle - both have their advantages.
(edit: I meant WiFi-only Kindle)
my partner's dx, on the other hand, had no problems whatsoever.