Debian switched back to GNOME in September over reasons dealing with accessibility, systemd integration, and other factors when seeing what was the best fit to be the default for Debian 8 Jessie. However, now for platforms aside from x86 and x86_64, Xfce has returned to the default over poor experiences in using the GNOME Shell.
The GNOME Shell depends on a suitable OpenGL driver stack for support, but in the ARM world the OpenGL driver support varies on the Linux (non-Android) desktop. Most Debian ARM platforms and Debian for other architectures is left using software acceleration (LLVMpipe) due to inadequate hardware or driver coverage. For this reason, Debian for non-x86/x86_64 architectures is returning to the Xfce world.
Joey Hess updated the tasksel Git today, "Gnome only works on i386 and amd64, so default to xfce on other arches."
The "GNOME only works on i386 and AMD64" come down to this bug report about OpenGL driver issues with GNOME Shell running on other platforms where the GL drivers aren't as good as the common Nouveau/Radeon/Intel open-source drivers and proprietary AMD/NVIDIA Linux graphics drivers.