The developers have said that this is not a final decision and they might return to GNOME
Debian developers have made a very important decision, namely to adopt the Xfce desktop environment and to ditch GNOME for Debian 8.0 “Jessie,” at least for the time being.The announcement regarding the new “look” of Debian 8.0 has been made by Joey Hess, one of the developers. He also said that this wasn’t a final measure and that they might return to GNOME before the distribution gets frozen.
“This will be re-evaluated before jessie is frozen (the switch to Xfce). The evaluation will start around the point of DebConf (August 2014). If at that point gnome looks like a better choice, it'll go back as the default,” he said in the mailing list.
There are a few reasons for this decision, and all of them are pretty sound: the accessibility support is much better in Xfce, the new desktop environment is a lot more reminiscent of the old GNOME 2, and the distribution still needs to fit on a CD.
The release date for Debian 8.0 hasn't been settled yet, but it should arrive by the end of 2015.