Since all desktop environments in Debian are maintained with equal commitments and you can install any of them easily, change of "default" desktop environment is a rather symbolic gesture.
Technically, the reason for the change is that GNOME won't fit in a CD without a diet and Xfce will. I am not sure how many people install Debian with CD (not DVD, not USB, not network install, etc.) though...
One point would be that if you're setting up rackmount server hardware, it still tends to come with a CD drive rather than DVD. That's one of Debian's primary purposes nowadays, given that Ubuntu has eaten most of its desktop market-share away.
From whom? Dell doesn't, to my knowledge, offer any CD drives. It's DVD or nothing; we usually choose the latter due to iDRAC's virtual CD/DVD capabilities anyway.
The last few rack-mounted servers we bought came with DVD writers, which seemed a little redundant to me. In our case, we only ever use the DVD drive once, to install CentOS, and then it's closed up and never used again. I can't imagine any situation where you'd want to write a DVD using a server...
Technically, the reason for the change is that GNOME won't fit in a CD without a diet and Xfce will. I am not sure how many people install Debian with CD (not DVD, not USB, not network install, etc.) though...