Why was the Due retired?
"Why was the Due retired?" - It is quite interesting that Arduino.CC never explained that. One would think, that a professional company or organisation places a notice in advance with at least a short explanation as to why.

Most of the Arduinos, that have been available on both projects (CC and ORG) have suddenly and without further notice or explanation been retired on CC (the exceptions being Uno and Mega 2560, the cash-cows), but are still alive on ORG.
It would seem, that either the "divorce" is proceeding or that the copyright case at the US court is not going as smoothly as Banzi hoped. Well, he could repeatedly "bend the truth" by claiming Arduino being completely his idea and design in front of the press (that never bothered to check the obvious facts), but laying such a claim at court is borderline criminal. (see also: arduinohistory.github.io and make your own opinion)
CC has still the biggest chunk of the overall community and also serves as the central nexus for developers. But it seems there is a lot of movement... some people move to Wiring (from which the Arduino IDE was forked), others to 3rd Party sites and yet others try out different  and newly emerging products (eg. Adafruits MCUs and Pi or the Renesas GR series).
ORG on the other hand seems to become the bigger (Arduino branded) hardware supplier and developer. CC has apparently out-sourced this to Intel and tries to live of royalties and "certification" fees (which is the central point where Banzi and Martino started to heavily and openly disagree).

"Why was the Due retired?" - Unless CC doesn't give an explanation, there is no real answer. A lack of interest is certainly not the reason.