The Xorg server no longer needs to be run as root, for security
reasons. See /usr/share/doc/xserver-xorg-core/NEWS.Debian.gz

xorg-server (2:1.17.3-1) unstable; urgency=medium

  The Xorg server is no longer setuid root by default.  This change reduces the
  risk of privilege escalation due to X server bugs, but has some side effects:

  * it relies on logind and libpam-systemd
  * it relies on a kernel video driver (so the userspace component doesn't
    touch the hardware directly)
  * it needs X to run on the virtual console (VT) it was started from
  * it changes the location for storing the Xorg log from /var/log/ to
    ~/.local/share/xorg/

  On systems where those are not available, the new xserver-xorg-legacy package
  is needed to allow X to run with elevated privileges.  See the
  Xwrapper.config(5) manual page for configuration details.