NASA gunshy?
"NASA wants astronauts on space-shuttle flights to be able to inspect and, if necessary, repair any damage to the shuttle's heat tiles and other components of its heat-protection system. The loss of heat shielding led to the Columbia disaster, which killed all seven astronauts aboard.
Such inspection and repair are much easier if the shuttle is docking at the International Space Station. As it happens, the space station is part of the itinerary for all currently scheduled missions except the ones to the Hubble. The shuttle can't carry enough fuel to visit both on the same mission."