I too am an admirer of the ATV and wish it had been developed further. I remember watching the ATV-1 "Jules Verne" mission at JSC and the "Houston, there's an X-wing requesting permission to dock" jokes1.
The official explanation is obsolescence of components.
"If we wanted to reopen production lines, there is a significant obsolescence problem at the equipment [and] component level," Chesson told Spaceflight Now.
Bob Chesson is/was "a senior advisor in ESA's human spaceflight directorate."
In a sense the ATV may live on as the precursor of the Orion service module.
The Orion service module, which provides propulsion, power, thermal control and elements of the life-support system for Orion, is based on ESA’s Automated Transfer Vehicle (ATV) cargo freighter. ESA had been reimbursing NASA for Europe’s 8 percent share of space station common operating charges by launching five ATV vehicles, which paid Europe’s station bills until 2017. The Orion service module completes this obligation through 2020.
I am not sure how this plays with the "obsolescence of components" rationale however.
1 The ATV docked with the Russian side of the ISS, but hey, it's a joke.