Why RHEL5 working pyGTK `%pre` script is not getting executed when used as `%pre` script in RHEL6 kickstart installation?

Issue

• Have a pyGTK %pre script which executes successfully in Red Hat Enterprise Linux 5 kickstart installations.
• When this script is put in a kickstart file as %pre script in Red Hat Enterprise Linux 6 installation, it doesn't get executed as expected and throws the following error.

Traceback (most recent call last):
  File "/tmp/ks_prekickstart_modified.py", line 6, in <module>
    import gtk
  File "/usr/lib64/python2.6/site-packages/gtk-2.0/gtk/__init__.py", line 64, in <module>
    _init()
  File "/usr/lib64/python2.6/site-packages/gtk-2.0/gtk/__init__.py", line 52, in _init
    _gtk.init_check()
RuntimeError: could not open display

• What changed between Red Hat Enterprise Linux 5 and 6 which is causing this failure?