Fuel would be a major issue.

Each pound of weight costs thousands of dollars to lift into space (making the entire shuttle program horribly inneficient, but that's another story). Carrying extra fuel to change orbits means not just carrying the fuel to change orbits, but the fuel to carry that fuel into orbit.

It's also a question of launch trajectory.

In many ways the space shuttle is very basic technology (compared to the stuff of science fiction), it's ability to maneuver is limited.
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When an object such as a space station or telescope is put into orbit many factors are involved in determining what it's trajectory and orbit are going to be. For the Hubble when it was put into orbit it didn't have a set trajectory for almost 3 years. It was based on its PoV for our solar system. The Hubble spacecraft has enough fuel to make course corrections and minute adjustments based on a timetable and set schedule, Its orbit is not fixed for all time and it is constantly being changed and adjusted. The orbit height is based on what the function of the craft is, for Hubble it was as I said put into orbit high enough where ozone, light pollution and surface glare wouldn't effect the visibility of objects in our solar system. Since its launch in april of 1990 the telescope's orbit has in fact been changed/corrected in every one of its 5 servicing missions to better optimize view of our visible universe.
The reason the shuttle(s) had to go to the Hubble telescope and not the
other way around was simply because the telescope HAS a set trajectory and is far more time consuming and less practical to move the Hubble to us as opposed to launching a small manned spacecraft to it.
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