Because Atlantis will be in Hubble's orbit, higher and farther than the ISS, NASA deems it necessary to have a backup mission capable of reaching the Hubble repair crew should anything prevent Atlantis from attempting a dangerous de-orbit. This is now standard procedure since the loss of the orbiter Columbia and her 7-astronaut crew in 2003. Unfortunately, mission planners realize that the shuttle doesn't have enough fuel to make the trip from Hubble's orbit to the ISS in case the crew needs to evacuate the shuttle. Chalk this one up to limiting a Shuttle's mission profile during design.
After the completion of STS125, Endeavor will be moved again, to primary launch pad 39A in preparation for its own mission STS127 to the ISS on June 13. Pad 39B will continue to be redeveloped into the primary launch pad for the new Constellation program which replaces the shuttles.
STS125 becomes an important final milestone in the shuttle program and in NASA space history. Last mission to have 2 shuttles on the pads at the same time, and last repair mission to the venerable Hubble space telescope. Hubble is destined to be replaced by the Next Generation Telescope, named after James Webb (former NASA director). With 5 spacewalks scheduled, this will be a memorable one for those of us who love being space fans.